[MCA] Introducing incremental SourceMgr and resumable pipeline

The new resumable mca::Pipeline capability introduced in this patch
allows users to save the current state of pipeline and resume from the
very checkpoint.
It is better (but not require) to use with the new IncrementalSourceMgr,
where users can add mca::Instruction incrementally rather than having a
fixed number of instructions ahead-of-time.

Note that we're using unit tests to test these new features. Because
integrating them into the `llvm-mca` tool will make too many churns.

Differential Revision: https://reviews.llvm.org/D127083