[coroutines] Add lexer support for co_await, co_yield, and co_return keywords.

Add -fcoroutines flag (just for -cc1 for now) to enable the feature. Early
indications are that this will be part of -std=c++1z.

llvm-svn: 250980

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1536,6 +1536,7 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   Opts.RTTIData = Opts.RTTI && !Args.hasArg(OPT_fno_rtti_data);
   Opts.Blocks = Args.hasArg(OPT_fblocks);
   Opts.BlocksRuntimeOptional = Args.hasArg(OPT_fblocks_runtime_optional);
+  Opts.Coroutines = Args.hasArg(OPT_fcoroutines);
   Opts.Modules = Args.hasArg(OPT_fmodules);
   Opts.ModulesStrictDeclUse = Args.hasArg(OPT_fmodules_strict_decluse);
   Opts.ModulesDeclUse =