[CMake] Add targets for generating coverage reports

This is a pretty small bit of CMake goop to generate code coverage
reports. I always forget the right script invocation and end up
fumbling around too much.

Wouldn't it be great to have targets that "Just Work"?

Well, I thought so.

At present this only really works correctly for LLVM, but I'll extend
it in subsequent patches to work for subprojects.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D109019