Handle char{8,16,32} and wchar_t in ASTContext::getIntegerRank()

They were previously not handled, causing the llvm_unreachable with
"getIntegerRank(): not a built-in integer" message to be hit.

The test case is derived from how we hit it in Chromium
(crbug.com/1396142). I tried to come up with something more direct, but
this is the best I could find.

Differential revision: https://reviews.llvm.org/D139428