Add an override to StringRef::getAsInteger which parses into an APInt.

Add an override to StringRef::getAsInteger which parses into an APInt.
It gets its own implementation totally divorced from the (presumably
performance-sensitive) routines which parse into a uint64_t.

Add APInt::operator|=(uint64_t), which is situationally much better than
using a full APInt.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97381 91177308-0d34-0410-b5e6-96231b3b80d8