[Support] make countLeadingZeros() and countTrailingZeros() return unsigned
This matches countLeadingOnes() and countTrailingOnes(), and
APInt's countLeadingZeros() and countTrailingZeros().
(as well as __builtin_clzll())
When committing changes, please add something like
Differential Revision: https://reviews.llvm.org/D60823
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