[llvm-exegesis] Finish plumbing the `Config` field.

[llvm-exegesis] Finish plumbing the `Config` field.

Summary:
Right now there are no snippet generators that emit the `Config` Field,
but I plan to add it to investigate LEA operands for PR32326.

What was broken was:
 - `Config` Was not propagated up until the BenchmarkResult::Key.
 - Clustering should really consider different configs as measuring
 different things, so we should stabilize on (Opcode, Config) instead of
 just Opcode.

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits, lebedev.ri

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68629

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374031 91177308-0d34-0410-b5e6-96231b3b80d8