Fix the hasType() AST matcher to not assert when the QualType is invalid.

There's not a particularly good way to test this with the AST matchers unit tests because the only way to get an invalid type (that I can devise) involves creating parse errors, which the test harness always treats as a failure. Instead, a clang-tidy test case will be added in a follow-up commit based on the original bug report.

llvm-svn: 326604