Add relational benchmark against a string constant.

Summary:
Add relational benchmark against a string constant.
These can potentially trigger inlining of the operations. We want to
benchmark that.

Reviewers: EricWF

Subscribers: christof, jdoerfert, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D59512

llvm-svn: 356680