[clang] Fix absolute file paths with -fdebug-prefix-map

[keith]

Summary

Previously if you passed an absolute path to clang, where only part of
the path to the file was remapped, it would result in the file's DIFile
being stored with a duplicate path, for example:

!DIFile(filename: "./ios/Sources/bar.c", directory: "./ios/Sources")

This change handles absolute paths, specifically in the case they are
remapped to something relative, and uses the dirname for the directory,
and basename for the filename.

This also adds a test verifying this behavior for more standard uses as
well.

[probinson]

I suspect this is not the only way for directories and filenames to become confused, but it certainly helps. LGTM.

[keith]

https://reviews.llvm.org/D111587

clang/lib/CodeGen/CGDebugInfo.cpp

if (std::distance(llvm::sys::path::begin(CurDir), CurDirIt) == 1) {
       Dir = {};
       File = RemappedFile;
     } else {
       for (; FileIt != FileE; ++FileIt)
         llvm::sys::path::append(FileBuf, *FileIt);
       Dir = DirBuf;
       File = FileBuf;
     }
+  } else if (llvm::sys::path::is_absolute(FileName)) {
+    Dir = llvm::sys::path::parent_path(RemappedFile);
+    File = llvm::sys::path::filename(RemappedFile);
   } else {
     Dir = CurDir;
     File = RemappedFile;
   }
   llvm::DIFile *F = DBuilder.createFile(File, Dir, CSInfo, Source);
   DIFileCache[FileName.data()].reset(F);
   return F;
 }

clang/test/CodeGen/relative-debug-prefix-map.c

+// RUN: mkdir -p %t.nested/dir && cd %t.nested/dir
+// RUN: cp %s %t.nested/dir/main.c
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%t.nested=. %t.nested/dir/main.c -emit-llvm -o - | FileCheck %s
+//
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=. %s -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-DIRECT
+//
+// RUN: cd %p
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-compilation-dir=. relative-debug-prefix-map.c -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-DIRECT
+
+// CHECK: !DIFile(filename: "main.c", directory: "./dir")
+// CHECK-DIRECT: !DIFile(filename: "relative-debug-prefix-map.c", directory: ".")
+
+int main(int argc, char **argv) {
+  (void)argc;
+  (void)argv;
+  return 0;
+}