Sema: show shift result in hexadecimal

Change the output for -Wshift-overflow and
-Wshift-sign-overflow to an unsigned hexadecimal. It makes
more sense for looking at bits than a signed decimal does.
Also, change the diagnostic's wording from "overrides"
to "sets".

This uses a new optional argument in APInt::toString()
that adds the '0x' prefix to hexademical numbers.

This fixes PR 9651.

Patch by nobled@dreamwidth.org!

llvm-svn: 133033