[clang] Replaced some manual pointer tagging with llvm::PointerIntPair.

There is no functional change here (hence no new tests). The only change
is to replace a couple uintptr_t members with llvm::PointerIntPair<> to
clean up the code, making it more readable and less error prone.

This cleanup highlighted that the old code was effectively casting away
const. This is fixed by changing some function signatures.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D98889