Add release_debug target

[lyuma]

No description provided.

[BastiaanOlij]

I've never really understood the need for both release debug and debug, I would just add the switch to debug, it's always supposed to have the debug symbols, actually surprised we got that switch wrong.

[lyuma]

I have split off the bugfix for debug symbols in the debug target, into #102 -- However, I think this PR is still relevant.

The use-case for a release_debug target is to build for release, while also generating a .pdb file which could be used for debugging crash reports sent by users.

There are some good arguments to be made here: https://stackoverflow.com/a/5457250

I will say, CI systems are often linux-based and will hit the use_llvm=yes case, which does already generate debug symbols on release. So the value of this PR is minimal in practice.

It may also make sense to just change the release to generate the .pdb file, to match the behavior of other platforms.