PR18234: Mark a tag definition as invalid early if it appears in a
type-specifier in C++. Some checks will assert in this case otherwise (in
particular, the access specifier may be missing if this happens inside a class
definition, due to a violation of an AST invariant).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198721 91177308-0d34-0410-b5e6-96231b3b80d8