[Frontend] Remove unused FileMgr in pp arg parse

[modocache]

Summary

A FIXME added 8 years ago (2010) in https://reviews.llvm.org/rL118203
mentioned that a FileManager should not need to be used when parsing
preprocessor arguments. In fact, its only use was removed 6 years ago
(2012), in https://reviews.llvm.org/rL166452. Remove the unused
variable and the obsolete FIXME.

Test Plan: check-clang

[v.g.vassilev]

LGTM!

[modocache]

Great, thanks for the review, @v.g.vassilev!

lib/Frontend/CompilerInvocation.cpp

static bool isStrictlyPreprocessorAction(frontend::ActionKind Action) {
   case frontend::RewriteMacros:
   case frontend::RunPreprocessorOnly:
     return true;
   }
   llvm_unreachable("invalid frontend action");
 }
 
 static void ParsePreprocessorArgs(PreprocessorOptions &Opts, ArgList &Args,
-                                  FileManager &FileMgr,
                                   DiagnosticsEngine &Diags,
                                   frontend::ActionKind Action) {
   using namespace options;
   Opts.ImplicitPCHInclude = Args.getLastArgValue(OPT_include_pch);
   Opts.ImplicitPTHInclude = Args.getLastArgValue(OPT_include_pth);
   if (const Arg *A = Args.getLastArg(OPT_token_cache))
       Opts.TokenCache = A->getValue();
   else
bool CompilerInvocation::CreateFromArgs(CompilerInvocation &Res,
 
   // FIXME: Override value name discarding when asan or msan is used because the
   // backend passes depend on the name of the alloca in order to print out
   // names.
   Res.getCodeGenOpts().DiscardValueNames &=
       !LangOpts.Sanitize.has(SanitizerKind::Address) &&
       !LangOpts.Sanitize.has(SanitizerKind::Memory);
 
-  // FIXME: ParsePreprocessorArgs uses the FileManager to read the contents of
-  // PCH file and find the original header name. Remove the need to do that in
-  // ParsePreprocessorArgs and remove the FileManager
-  // parameters from the function and the "FileManager.h" #include.
-  FileManager FileMgr(Res.getFileSystemOpts());
-  ParsePreprocessorArgs(Res.getPreprocessorOpts(), Args, FileMgr, Diags,
+  ParsePreprocessorArgs(Res.getPreprocessorOpts(), Args, Diags,
                         Res.getFrontendOpts().ProgramAction);
   ParsePreprocessorOutputArgs(Res.getPreprocessorOutputOpts(), Args,
                               Res.getFrontendOpts().ProgramAction);
 
   // Turn on -Wspir-compat for SPIR target.
   llvm::Triple T(Res.getTargetOpts().Triple);
   auto Arch = T.getArch();
   if (Arch == llvm::Triple::spir || Arch == llvm::Triple::spir64) {