[clang][pp] Handle attributes defined by plugin in __has_attribute

When using attributes by plugins (both in clang and clang-tidy) the
preprocessor functions `__has_attribute`, `__has_c_attribute`,
`__has_cpp_attribute` still returned 0.

That problem is fixed by having the "hasAttribute" function also check
if any of the plugins provide that attribute.

This also adds C2x spelling to the example plugin for attributes so that
`__has_c_attribute` can be tested.

Differential Revision: https://reviews.llvm.org/D144405