[Coverage] Map regions from system headers

Originally, the following commit removed mapping coverage regions for system headers:
https://github.com/llvm/llvm-project/commit/93205af066341a53733046894bd75c72c99566db

It might be viable and useful to collect coverage from system headers in some systems.
This patch adds --system-headers-coverage option (disabled by default) to enable
collecting coverage from system headers.

Differential Revision: https://reviews.llvm.org/D143304