If char/short are shorter than int, do not use U as suffix for

If char/short are shorter than int, do not use U as suffix for
constants. Comparing int against a constant of the given type like
UINT8_MAX will otherwise force a promotion to unsigned int, which is
typically not expected.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213301 91177308-0d34-0410-b5e6-96231b3b80d8