[libclang] Support querying whether a declaration is invalid

This is useful for e.g. highlighting purposes in an IDE.

Note: First version of this patch was reverted due to failing tests in
opencl-types.cl with -target ppc64le-unknown-linux. These tests are
adapted now.

Patch by Nikolai Kosjar.

Differential Revision: https://reviews.llvm.org/D40072

llvm-svn: 321794