[clang] Don't require FileManager for creating an output file (llvm#164665)

Conceptually, the `CompilerInstance` doesn't need an instance of the
`FileManager` to create an output file. This PR enables that, removing
an edge-case in `cc1_main()`.

Author: jansvoboda11

clang/include/clang/Basic/FileManager.h

@@ -287,8 +287,12 @@ class FileManager : public RefCountedBase<FileManager> {
   /// If path is not absolute and FileSystemOptions set the working
   /// directory, the path is modified to be relative to the given
   /// working directory.
-  /// \returns true if \c path changed.
-  bool FixupRelativePath(SmallVectorImpl<char> &path) const;
+  /// \returns true if \c Path changed.
+  bool FixupRelativePath(SmallVectorImpl<char> &Path) const {
+    return fixupRelativePath(FileSystemOpts, Path);
+  }
+  static bool fixupRelativePath(const FileSystemOptions &FileSystemOpts,
+                                SmallVectorImpl<char> &Path);
 
   /// Makes \c Path absolute taking into account FileSystemOptions and the
   /// working directory option.

clang/lib/Basic/FileManager.cpp

@@ -474,16 +474,17 @@ OptionalFileEntryRef FileManager::getBypassFile(FileEntryRef VF) {
   return FileEntryRef(*Insertion.first);
 }
 
-bool FileManager::FixupRelativePath(SmallVectorImpl<char> &path) const {
-  StringRef pathRef(path.data(), path.size());
+bool FileManager::fixupRelativePath(const FileSystemOptions &FileSystemOpts,
+                                    SmallVectorImpl<char> &Path) {
+  StringRef pathRef(Path.data(), Path.size());
 
   if (FileSystemOpts.WorkingDir.empty()
       || llvm::sys::path::is_absolute(pathRef))
     return false;
 
   SmallString<128> NewPath(FileSystemOpts.WorkingDir);
   llvm::sys::path::append(NewPath, pathRef);
-  path = NewPath;
+  Path = NewPath;
   return true;
 }
 

clang/lib/Frontend/CompilerInstance.cpp

@@ -882,7 +882,7 @@ CompilerInstance::createOutputFileImpl(StringRef OutputPath, bool Binary,
            "File Manager is required to fix up relative path.\n");
 
     AbsPath.emplace(OutputPath);
-    FileMgr->FixupRelativePath(*AbsPath);
+    FileManager::fixupRelativePath(getFileSystemOpts(), *AbsPath);
     OutputPath = *AbsPath;
   }
 

clang/tools/driver/cc1_main.cpp

@@ -313,17 +313,6 @@ int cc1_main(ArrayRef<const char *> Argv, const char *Argv0, void *MainAddr) {
   llvm::TimerGroup::clearAll();
 
   if (llvm::timeTraceProfilerEnabled()) {
-    // It is possible that the compiler instance doesn't own a file manager here
-    // if we're compiling a module unit. Since the file manager are owned by AST
-    // when we're compiling a module unit. So the file manager may be invalid
-    // here.
-    //
-    // It should be fine to create file manager here since the file system
-    // options are stored in the compiler invocation and we can recreate the VFS
-    // from the compiler invocation.
-    if (!Clang->hasFileManager())
-      Clang->createFileManager();
-
     if (auto profilerOutput = Clang->createOutputFile(
             Clang->getFrontendOpts().TimeTracePath, /*Binary=*/false,
             /*RemoveFileOnSignal=*/false,