Allow remapping Clang module skeleton CU references with -fdebug-prefix-map

[aprantl]

Summary

rdar://problem/55685132

clang/lib/CodeGen/CGDebugInfo.cpp

if (CreateSkeletonCU && IsRootModule && !Mod.getASTFile().empty()) {
     // but LLVM detects skeleton CUs by looking for a non-zero DWO id.
     // We use the lower 64 bits for debug info.
     uint64_t Signature =
         Mod.getSignature()
             ? (uint64_t)Mod.getSignature()[1] << 32 | Mod.getSignature()[0]
             : ~1ULL;
     llvm::DIBuilder DIB(CGM.getModule());
     SmallString<0> PCM;
-    if (!llvm::sys::path::is_absolute(PCM))
+    if (!llvm::sys::path::is_absolute(Mod.getASTFile()))
       PCM = Mod.getPath();
     llvm::sys::path::append(PCM, Mod.getASTFile());
-    StringRef CompDir = getCurrentDirname();
+    std::string RemappedPCM = remapDIPath(PCM);
+    StringRef RelativePCM(RemappedPCM);
+    StringRef CompDir = TheCU->getDirectory();
+    if (RelativePCM.consume_front(CompDir))
+      RelativePCM.consume_front(llvm::sys::path::get_separator());
     DIB.createCompileUnit(TheCU->getSourceLanguage(),
                           // TODO: Support "Source" from external AST providers?
                           DIB.createFile(Mod.getModuleName(), CompDir),
-                          TheCU->getProducer(), false, StringRef(), 0, PCM,
+                          TheCU->getProducer(), false, StringRef(), 0, RelativePCM,
                           llvm::DICompileUnit::FullDebug, Signature);
     DIB.finalize();
   }
 
   llvm::DIModule *Parent =
       IsRootModule ? nullptr
                    : getOrCreateModuleRef(ASTSourceDescriptor(*M->Parent),
                                           CreateSkeletonCU);

clang/test/Modules/debug-info-moduleimport.m

 // RUN: rm -rf %t
-// RUN: %clang_cc1 -debug-info-kind=limited -fmodules -DGREETING="Hello World" -UNDEBUG -fimplicit-module-maps -fmodules-cache-path=%t %s -I %S/Inputs -isysroot /tmp/.. -I %t -emit-llvm -o - | FileCheck %s --check-prefix=NOIMPORT
+// RUN: %clang_cc1 -debug-info-kind=limited -fmodules \
+// RUN:     -DGREETING="Hello World" -UNDEBUG \
+// RUN:     -fimplicit-module-maps -fmodules-cache-path=%t %s \
+// RUN:     -I %S/Inputs -isysroot /tmp/.. -I %t -emit-llvm -o - \
+// RUN:     | FileCheck %s --check-prefix=NOIMPORT
 
 // NOIMPORT-NOT: !DIImportedEntity
 // NOIMPORT-NOT: !DIModule
 
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -debug-info-kind=limited -fmodules -DGREETING="Hello World" -UNDEBUG -fimplicit-module-maps -fmodules-cache-path=%t %s -I %S/Inputs -isysroot /tmp/.. -I %t -emit-llvm -debugger-tuning=lldb -o - | FileCheck %s
+// RUN: %clang_cc1 -debug-info-kind=limited -fmodules \
+// RUN:    -DGREETING="Hello World" -UNDEBUG \
+// RUN:    -fimplicit-module-maps -fmodules-cache-path=%t %s \
+// RUN:    -I %S/Inputs -isysroot /tmp/.. -I %t -emit-llvm \
+// RUN:    -debugger-tuning=lldb -o - | FileCheck %s
 
 // CHECK: ![[CU:.*]] = distinct !DICompileUnit
 // CHECK-SAME:  sysroot: "/tmp/..")
 @import DebugObjC;
 // CHECK: !DIImportedEntity(tag: DW_TAG_imported_declaration, scope: ![[CU]],
 // CHECK-SAME:              entity: ![[MODULE:.*]], file: ![[F:[0-9]+]],
 // CHECK-SAME:              line: [[@LINE-3]])
 // CHECK: ![[MODULE]] = !DIModule(scope: null, name: "DebugObjC",
 // CHECK-SAME:  configMacros: "\22-DGREETING=Hello World\22 \22-UNDEBUG\22",
 // CHECK-SAME:  includePath: "{{.*}}test{{.*}}Modules{{.*}}Inputs"
 // CHECK: ![[F]] = !DIFile(filename: {{.*}}debug-info-moduleimport.m
 
-// RUN: %clang_cc1 -debug-info-kind=limited -fmodules -fimplicit-module-maps -fmodules-cache-path=%t \
-// RUN:   %s -I %S/Inputs -isysroot /tmp/.. -I %t -emit-llvm -o - \
-// RUN:     | FileCheck %s --check-prefix=NO-SKEL-CHECK
+// RUN: %clang_cc1 -debug-info-kind=limited -fmodules -fimplicit-module-maps \
+// RUN:   -fmodules-cache-path=%t %s -I %S/Inputs -isysroot /tmp/.. -I %t \
+// RUN:   -emit-llvm -o - | FileCheck %s --check-prefix=NO-SKEL-CHECK
 // NO-SKEL-CHECK: distinct !DICompileUnit
 // NO-SKEL-CHECK-NOT: distinct !DICompileUnit
 
-// RUN: %clang_cc1 -debug-info-kind=limited -fmodules -fimplicit-module-maps -fmodules-cache-path=%t \
+// RUN: %clang_cc1 -debug-info-kind=limited -fmodules -fimplicit-module-maps \
+// RUN:   -fmodules-cache-path=%t -fdebug-prefix-map=%t=/MODULE-CACHE \
 // RUN:   -fmodule-format=obj -dwarf-ext-refs \
 // RUN:   %s -I %S/Inputs -isysroot /tmp/.. -I %t -emit-llvm -o - \
 // RUN:     | FileCheck %s --check-prefix=SKEL-CHECK
 // SKEL-CHECK: distinct !DICompileUnit({{.*}}file: ![[CUFILE:[0-9]+]]
 // SKEL-CHECK: ![[CUFILE]] = !DIFile({{.*}}directory: "[[COMP_DIR:.*]]"
-// SKEL-CHECK: distinct !DICompileUnit({{.*}}file: ![[DWOFILE:[0-9]+]]{{.*}}dwoId
+// SKEL-CHECK: distinct !DICompileUnit({{.*}}file: ![[DWOFILE:[0-9]+]]{{.*}}splitDebugFilename: "/MODULE-CACHE{{.*}}dwoId
 // SKEL-CHECK: ![[DWOFILE]] = !DIFile({{.*}}directory: "[[COMP_DIR]]"

clang/test/PCH/debug-info-pch-path.c

 //
 //
 // CHECK-REL-NODIR: !DICompileUnit
 // CHECK-REL-NODIR-SAME:           file: ![[C:[0-9]+]]
 // CHECK-REL-NODIR-NOT: dwoId
 // CHECK-REL-NODIR: ![[C]] = !DIFile({{.*}}directory: "[[DIR:.*]]"
 // CHECK-REL-NODIR: !DICompileUnit(
 // CHECK-REL-NODIR-SAME:           file: ![[PCH:[0-9]+]]
-// CHECK-REL-NODIR-SAME:           splitDebugFilename: "{{.*}}PCH{{.*}}prefix.pch"
+// CHECK-REL-NODIR-SAME:           splitDebugFilename: "prefix.pch"
 // CHECK-REL-NODIR: ![[PCH]] = !DIFile({{.*}}directory: "[[DIR]]
 
 // ---------------------------------------------------------------------
 // Relative PCH in a subdirectory.
 // ---------------------------------------------------------------------
 //
 // RUN: mkdir pchdir
 // RUN: %clang_cc1 -fmodule-format=obj -emit-pch                \
 // RUN:     -triple %itanium_abi_triple                         \
 // RUN:     -o pchdir/prefix.pch %S/debug-info-limited-struct.h
 //
 // RUN: %clang_cc1 -debug-info-kind=standalone                  \
 // RUN:     -dwarf-ext-refs -fmodule-format=obj                 \
 // RUN:     -triple %itanium_abi_triple                         \
 // RUN:     -include-pch pchdir/prefix.pch %s -emit-llvm -o %t.rel.ll %s
 // RUN: cat %t.rel.ll | FileCheck %s --check-prefix=CHECK-REL
 
 // CHECK-REL: !DICompileUnit
 // CHECK-REL-SAME:           file: ![[C:[0-9]+]]
 // CHECK-REL-NOT: dwoId
 // CHECK-REL: ![[C]] = !DIFile({{.*}}directory: "[[DIR:.*]]"
 // CHECK-REL: !DICompileUnit(
 // CHECK-REL-SAME:           file: ![[PCH:[0-9]+]]
-// CHECK-REL-SAME:           splitDebugFilename: "[[DIR]]{{.*}}pchdir{{.*}}prefix.pch"
+// CHECK-REL-SAME:           splitDebugFilename: "pchdir{{.*}}prefix.pch"
 // CHECK-REL: ![[PCH]] = !DIFile({{.*}}directory: "[[DIR]]"
 
 // ---------------------------------------------------------------------
 // Absolute PCH.
 // ---------------------------------------------------------------------
 //
 // RUN: %clang_cc1 -fmodule-format=obj -emit-pch                \
 // RUN:     -triple %itanium_abi_triple                         \
 // RUN:     -o %t/prefix.pch %S/debug-info-limited-struct.h
 //
 // RUN: %clang_cc1 -debug-info-kind=standalone                  \
 // RUN:     -dwarf-ext-refs -fmodule-format=obj                 \
 // RUN:     -triple %itanium_abi_triple                         \
 // RUN:     -include-pch %t/prefix.pch %s -emit-llvm -o %t.abs.ll %s
 // RUN: cat %t.abs.ll | FileCheck %s --check-prefix=CHECK-ABS
 
 // CHECK-ABS: !DICompileUnit
 // CHECK-ABS-SAME:           file: ![[C:[0-9]+]]
 // CHECK-ABS-NOT: dwoId
 // CHECK-ABS: ![[C]] = !DIFile({{.*}}directory: "[[DIR:.*]]"
 // CHECK-ABS: !DICompileUnit(
 // CHECK-ABS-SAME:           file: ![[PCH:[0-9]+]]
-// CHECK-ABS-SAME:           splitDebugFilename: "[[DIR]]{{.*}}prefix.pch"
+// CHECK-ABS-SAME:           splitDebugFilename: "prefix.pch"
 // CHECK-ABS: ![[PCH]] = !DIFile({{.*}}directory: "[[DIR]]