Fix HashString's Bernstein hash to use unsigned chars, as is usually done.

Fix HashString's Bernstein hash to use unsigned chars, as is usually done.

Changes the hash result for strings containing characters
with values >= 128, such as UTF8 strings (not normal ASCII).

Changed mostly so we match other implementations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162882 91177308-0d34-0410-b5e6-96231b3b80d8