Create a runtime dedicated to coverage using IL only

[pjanotti]

This allows OpenCover to get source information for CoreLib (perhaps an issue to be investigated separately, since in theory it should have worked with the ni image and respective PDB). It also makes easier to experiment with tools that perform IL rewrite instead of relying on Profiler API.

Related to https://github.com/dotnet/corefx/issues/23588

[pjanotti]

FYI: Even after reviewed I will hold off merge until I'm able to fully test this in CoreFx (it takes a bit of time to run it when coverage is enabled).

[tarekgh]

@pjanotti don't block on my comments if @weshaggard and @vancem are ok with the changes. thanks for fixing this :-)

[pjanotti]

Dev box runs for coverage are looking good.

[pjanotti]

@weshaggard, please, take a look when you have a chance.

[weshaggard]

I asked a couple questions but the general approach looks good.

[pjanotti]

I had to add some explicit copying/deleting for the runtimeconfig.json since SupplementalTestData by default use hard links (we don't want to overwrite the one using the "default" shared test runtime). Planning to merge as soon as tests are passing.