Assert that Parser::ParseStructUnionBody is not called for C++ code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178631 91177308-0d34-0410-b5e6-96231b3b80d8
Enable use of _Static_assert inside structs and unions in C11 mode (as per C11 6.7.2.1p1).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178632 91177308-0d34-0410-b5e6-96231b3b80d8