[MCA] Add support for nested and overlapping region markers

This patch fixes PR41523
https://bugs.llvm.org/show_bug.cgi?id=41523

Regions can now nest/overlap provided that they have different names.
Anonymous regions cannot overlap.

Region end markers must specify the region name. The only exception is for when
there is only one user-defined region; in that particular case, the region end
marker doesn't need to specify a name.

Incorrect region end markers are no longer ignored. Instead, the tool reports an
error and we exit with an error code.

Added test cases to verify the new diagnostic error messages.

Updated the llvm-mca docs to reflect this feature change.

Differential Revision: https://reviews.llvm.org/D61676

llvm-svn: 360351