[flang] [clang] [flang][windows] Add option to link against specific MSVC CRT (PR #70833)

[Reid Kleckner via cfe-commits]

rnk wrote:

> Do we really need to have all 4 variants of the 3 fortran runtime libraries? That's a lot of complexity. Can we pare it down to just static/dynamic? It's also sometimes possible to generate code that works in both the static and dynamic context, depending on what is in those libraries. We don't create 4 variants of clang_rt.builtins, for examle.

>From glancing at the fortran runtime code, I think the answer is probably "no". There is too much C++ standard library usage. If you wish to avoid this build complexity, you may consider writing code in the STL-less style that is used for C++ code in the sanitizers in compiler-rt.

https://github.com/llvm/llvm-project/pull/70833