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.

Differential Revision: https://reviews.llvm.org/D113718

Author: adrian-prantl

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

@@ -5,6 +5,8 @@
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -isysroot %p -debugger-tuning=lldb | FileCheck %s -check-prefix CHECK-SYSROOT
 // RUN: %clang -g -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s
 // RUN: %clang -g -ffile-prefix-map=%p=/UNLIKELY_PATH/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"
 
@@ -42,3 +44,7 @@ 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: "")