[AST] RecoveryAST options should only be enabled for C++ only.

[hokein]

Summary

The flag should be off for non-c++ files even there is a "-frecovery-ast" in
the commandline.

[sammccall]

Hmm - why?
I thought the idea with the explicit -frecovery-ast flag was that we could turn it on even when it's not stable.
Isn't the idea that one day it will be stable for C, but not yet?

[hokein]

In D81452#2081838, @sammccall wrote:

Hmm - why?
I thought the idea with the explicit -frecovery-ast flag was that we could turn it on even when it's not stable.
Isn't the idea that one day it will be stable for C, but not yet?

oops, right. I think I was highly confused by the misleading results caused by my messed-up patches locally :(

clang/lib/Frontend/CompilerInvocation.cpp

if (Opts.NewAlignOverride && !llvm::isPowerOf2_32(Opts.NewAlignOverride)) {
     Diags.Report(diag::err_fe_invalid_alignment) << A->getAsString(Args)
                                                  << A->getValue();
     Opts.NewAlignOverride = 0;
   }
   Opts.ConceptSatisfactionCaching =
       !Args.hasArg(OPT_fno_concept_satisfaction_caching);
   if (Args.hasArg(OPT_fconcepts_ts))
     Diags.Report(diag::warn_fe_concepts_ts_flag);
+
+  // Recovery AST still heavily relies on dependent-type machinery, so C++ only
+  // for now.
   Opts.RecoveryAST =
-      Args.hasFlag(OPT_frecovery_ast, OPT_fno_recovery_ast, false);
+      Args.hasFlag(OPT_frecovery_ast, OPT_fno_recovery_ast, false) &&
+      Opts.CPlusPlus;
   Opts.RecoveryASTType =
-+      Args.hasFlag(OPT_frecovery_ast_type, OPT_fno_recovery_ast_type, false);
+      Args.hasFlag(OPT_frecovery_ast_type, OPT_fno_recovery_ast_type, false) &&
+      Opts.CPlusPlus;
+
   Opts.HeinousExtensions = Args.hasArg(OPT_fheinous_gnu_extensions);
   Opts.AccessControl = !Args.hasArg(OPT_fno_access_control);
   Opts.ElideConstructors = !Args.hasArg(OPT_fno_elide_constructors);
   Opts.MathErrno = !Opts.OpenCL && Args.hasArg(OPT_fmath_errno);
   Opts.InstantiationDepth =
       getLastArgIntValue(Args, OPT_ftemplate_depth, 1024, Diags);
   Opts.ArrowDepth =
       getLastArgIntValue(Args, OPT_foperator_arrow_depth, 256, Diags);