[MSVC Compat] Implement -EHc semantics

The -EHc flag implicitly adds a nothrow attribute to any extern "C"
function when exceptions are enabled.

llvm-svn: 261425
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 2e08227..cf4b5cf 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1688,6 +1688,7 @@
   Opts.ObjCExceptions = Args.hasArg(OPT_fobjc_exceptions);
   Opts.CXXExceptions = Args.hasArg(OPT_fcxx_exceptions);
   Opts.SjLjExceptions = Args.hasArg(OPT_fsjlj_exceptions);
+  Opts.ExternCNoUnwind = Args.hasArg(OPT_fexternc_nounwind);
   Opts.TraditionalCPP = Args.hasArg(OPT_traditional_cpp);
 
   Opts.RTTI = Opts.CPlusPlus && !Args.hasArg(OPT_fno_rtti);