Avoid appending the source directory to an absolute path

[aprantl]

Summary

When building a precompiled header in -fmodule-format=obj (i.e., `-gmodules) in an absolute path, the locig in CGDebugInfo::createCompileUnit would unconditionally append the source directory to the -main-file-name. This patch avoids that behavior for absolute paths.

rdar://problem/46045865

[JDevlieghere]

clang/lib/CodeGen/CGDebugInfo.cpp

541

Should we check if MainFileDir is absolute to, instead of just checking for .?

[JDevlieghere]

LGTM modulo the comment.

[aprantl]

clang/lib/CodeGen/CGDebugInfo.cpp

541

We might want to unconditionally call remove-dots instead.

clang/lib/CodeGen/CGDebugInfo.cpp

if (MainFileName.empty())
     MainFileName = "<stdin>";
 
   // The main file name provided via the "-main-file-name" option contains just
   // the file name itself with no path information. This file name may have had
   // a relative path, so we look into the actual file entry for the main
   // file to determine the real absolute path for the file.
   std::string MainFileDir;
   if (const FileEntry *MainFile = SM.getFileEntryForID(SM.getMainFileID())) {
-    MainFileDir = remapDIPath(MainFile->getDir()->getName());
-    if (MainFileDir != ".") {
+    MainFileDir = MainFile->getDir()->getName();
+    if (MainFileDir != "." && !llvm::sys::path::is_absolute(MainFileName)) {
       llvm::SmallString<1024> MainFileDirSS(MainFileDir);
       llvm::sys::path::append(MainFileDirSS, MainFileName);
       MainFileName = MainFileDirSS.str();
     }
     // If the main file name provided is identical to the input file name, and
     // if the input file is a preprocessed source, use the module name for
     // debug info. The module name comes from the name specified in the first
     // linemarker if the input is a preprocessed source.

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

+// REQUIRES: asserts
+
+// Modules:
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: cd %t
+
+// RUN: %clang_cc1 -fmodule-format=obj -emit-pch                \
+// RUN:     -triple %itanium_abi_triple                         \
+// RUN:     -fdebug-prefix-map=%t=BUILD                         \
+// RUN:     -fdebug-prefix-map=%S=SOURCE                        \
+// RUN:     -o %t/prefix.ll %S/debug-info-limited-struct.h      \
+// RUN:   -mllvm -debug-only=pchcontainer &>%t-container.ll
+// RUN: cat %t-container.ll | FileCheck %s
+
+// CHECK: distinct !DICompileUnit(
+// CHECK-SAME:                    language: DW_LANG_C99,
+// CHECK-SAME:                    file: ![[FILE:[0-9]+]],
+// CHECK: ![[FILE]] = !DIFile(
+// CHECK-SAME:                filename: "SOURCE/debug-info-limited-struct.h",
+// CHECK-SAME:                directory: "BUILD"
+