Add more benchmarks for std::string.

Summary:
Added benchmarks for Construct, Copy, Move, Destroy, Relationals and
Read. On the ones that matter, the benchmarks tests hot and cold data,
and opaque and transparent inputs.

Reviewers: EricWF

Subscribers: christof, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53825

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345611 91177308-0d34-0410-b5e6-96231b3b80d8