reland: [clang] Don't append the working directory to absolute paths

This fixes a bug that happens when using -fdebug-prefix-map to remap an
absolute path to a relative path. Since the path was absolute before
remapping, it is safe to assume that concatenating the remapped working
directory would be wrong.

This was originally submitted as https://reviews.llvm.org/D113718, but
reverted because when testing with dwarf 5 enabled, the tests were too
strict.

Differential Revision: https://reviews.llvm.org/D121663

Author: keith

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -444,7 +444,8 @@ CGDebugInfo::createFile(StringRef FileName,
       File = FileBuf;
     }
   } else {
-    Dir = CurDir;
+    if (!llvm::sys::path::is_absolute(FileName))
+      Dir = CurDir;
     File = RemappedFile;
   }
   llvm::DIFile *F = DBuilder.createFile(File, Dir, CSInfo, Source);

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

@@ -6,6 +6,9 @@
 // RUN: %clang -g -fdebug-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty -S -c %s -emit-llvm -o - | FileCheck %s
 // RUN: %clang -g -ffile-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty -S -c %s -emit-llvm -o - | FileCheck %s
 
+// RUN: %clang -g -fdebug-prefix-map=%p=./UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-REL
+// RUN: %clang -g -ffile-prefix-map=%p=./UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-REL
+
 #include "Inputs/stdio.h"
 
 int main(int argc, char **argv) {
@@ -40,3 +43,7 @@ void test_rewrite_includes(void) {
 // CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}Inputs{{/|\\\\}}stdio.h", directory: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty")
 // CHECK-COMPILATION-DIR-NOT: !DIFile(filename:
 // CHECK-SYSROOT: !DICompileUnit({{.*}}sysroot: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty"
+
+// CHECK-REL: !DIFile(filename: "./UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}",
+// CHECK-REL: !DIFile(filename: "./UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}Inputs/stdio.h",
+// CHECK-REL-SAME:    directory: ""