[Frontend] Remove unneeded -frtti check. NFC

-frtti is not a CC1 option. We just need to check -fno-rtti.

Author: MaskRay

clang/lib/Frontend/CompilerInvocation.cpp

@@ -4178,7 +4178,7 @@ bool CompilerInvocation::ParseLangArgs(LangOptions &Opts, ArgList &Args,
                    TargetCXXABI::usesRelativeVTables(T));
 
   // RTTI is on by default.
-  bool HasRTTI = Args.hasFlag(options::OPT_frtti, options::OPT_fno_rtti, true);
+  bool HasRTTI = !Args.hasArg(options::OPT_fno_rtti);
   Opts.OmitVTableRTTI =
       Args.hasFlag(options::OPT_fexperimental_omit_vtable_rtti,
                    options::OPT_fno_experimental_omit_vtable_rtti, false);