[llvm-exegesis] Add a Configuration object for Benchmark.

[llvm-exegesis] Add a Configuration object for Benchmark.

Summary: This is the first step to have the BenchmarkRunner create and measure many different configurations (different initial values for instance).

Reviewers: courbet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D47826

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334169 91177308-0d34-0410-b5e6-96231b3b80d8