[AST] Add a fast path to ConstantArrayType::getNumAddressingBits().

 - This fast path is almost 100% effective on real code, and lets us avoid
   multiple allocations of 128-bit APSInt objects in the common case.

 - As with any overflow-check-skipping-code, I'd appreciate someone double
   checking my logic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177183 91177308-0d34-0410-b5e6-96231b3b80d8