Test different combinations of options

[mgorny]

I think we should be testing all different combinations of options, e.g. <all -rtlib options> x <all -stdlib options> x ...

For example, on Gentoo -rtlib=libgcc -fsanitize=address works fine but -rtlib=compiler-rt -fsanitize=address fails due to missing unwinder linking.

I suppose we could solve this by adding tests inline but I think there is some way to parametrize tests in lit, i.e. use one test case with multiple variants of substitution.

[sylvestre]

@mgorny would you like to give it a try ? ;)