StringRef: Just forward StringRef::find to libc's memchr.

StringRef: Just forward StringRef::find to libc's memchr.

Modern libc's have an SSE version of memchr which is a lot faster than our
hand-rolled version. In the past I was reluctant to use it because Darwin's
memchr used a naive ridiculously slow implementation, but that has been fixed
some versions ago.

Should have zero functional impact.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232898 91177308-0d34-0410-b5e6-96231b3b80d8