Avoid calling mergeTypes in C++. I think these are the correct C++

alternatives, but please correct me if I'm wrong.

I eventually plan to assert in mergeTypes that we aren't in C++ mode 
because composite types are fundamentally not a part of C++. The 
remaining callers for code in the regression tests are 
Sema::WarnConflictingTypedMethods and CodeGenFunction::EmitFunctionProlog;
I'm not quite sure what the correct approach is for those callers.

llvm-svn: 71946