[CTU] Add triple/lang mismatch handling

Summary:
We introduce a strict policy for C++ CTU. It can work across TUs only if
the C++ dialects are the same. We neither allow C vs C++ CTU.  We do this
because the same constructs might be represented with different properties in
the corresponding AST nodes or even the nodes might be completely different (a
struct will be RecordDecl in C, but it will be a CXXRectordDecl in C++, thus it
may cause certain assertions during cast operations).

Reviewers: xazax.hun, a_sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Differential Revision: https://reviews.llvm.org/D55134

llvm-svn: 348610