[Support] make countLeadingZeros() countTrailingZeros() countLeadingOnes() and...

[Support] make countLeadingZeros() countTrailingZeros() countLeadingOnes() and countTrailingOnes() return unsigned

This matches APInt's versions of these functions, and there is no need for these to be size_t.

(as well as __builtin_clzll())

Differential Revision: https://reviews.llvm.org/D60823

llvm-svn: 362503