[clang] Fix signedness in vector bitcast evaluation

The included test case triggered a sign assertion on the result in
`Success()`.  This was caused by the APSInt created for a bitcast
having its signedness bit inverted.  The second APSInt constructor
argument is `isUnsigned`, so invert the result of
`isSignedIntegerType`.

Differential Revision: https://reviews.llvm.org/D95135