[llvm-mca] Fix duplicate symbols error

Parsing instruments and analysis regions causes us to see the same
labels two times since we parse the same file twice under the same
context.

This change creates a seperate context for instrument parsing
and another for analysis region parsing. I will post a follow up
commit once I get some free cycles to parse analysis regions and
instruments in one parsing pass under a single context.

Differential Revision: https://reviews.llvm.org/D149781