[llvm-mca] Fix JSON format for multiple regions

Instead of printing each region individually when using JSON format,
this patch creates a JSON object which is updated with the values of
each region, printing them at the end. New test is added for JSON output
with multiple regions.

Bug: https://bugs.llvm.org/show_bug.cgi?id=51008

Reviewed By: andreadb

Differential Revision: https://reviews.llvm.org/D105618