[llvm] [flang-rt] Remove hard-coded dependency on compiler-rt path on Windows (PR #150244)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> I meant more specifically that flang-rt exists to be linked into binaries compiled by flang, which already adds the dependency to compiler-rt as well. So flang-rt doesn't really need the dependency itself.

Non-Flang-compiled artifacts may also want to link to flang_rt.runtime, e.g. flang-rt as a dll (although not yet possible), C/C++ programs that #include <ISO_Fortran_binding.h> like those unittests.

> This way we can keep the logic for finding and linking compiler-rt all in one place in the flang driver, rather than trying to duplicate that logic in cmake.

And yet we duplicate them for the unittests?!? If it wasn't for `iso_fortran_env_impl.f90`, the Flang driver wouldn't be involved at all. Since it is, just `target_link_directories` to the compiler-rt location might work as well?

https://github.com/llvm/llvm-project/pull/150244