[llvm-cov] Add an option which maps the location of source directories on...

[llvm-cov] Add an option which maps the location of source directories on another machine to your local copies

Summary:
This patch adds the -path-equivalence option (example: llvm-cov show -path-equivalence=/origin/path,/local/path) which maps the source code path from one machine to another when using `llvm-cov show`. This is similar to the -filename-equivalence option, but doesn't require you to specify all the source files on the command line.

This allows you to generate the coverage data on one machine (e.g. in a CI system), and then use llvm-cov on another machine where you have the same code base on a different path.

Reviewers: vsk

Reviewed By: vsk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36391

llvm-svn: 310827