[CTU] Do not allow different CPP dialects in CTU

Summary:
If CPP dialects are different then return with error.

Consider this STL code:
  template<typename _Alloc>
    struct __alloc_traits
  #if __cplusplus >= 201103L
    : std::allocator_traits<_Alloc>
  #endif
    { // ...
    };
This class template would create ODR errors during merging the two units,
since in one translation unit the class template has a base class, however
in the other unit it has none.

Reviewers: xazax.hun, a_sidorin, r.stahl

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57906

llvm-svn: 355096