[clang] CodeGen: support static linking for libclosure (PR #125384)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I thought the Windows linker actually can handle statically linking dllimported symbols this (maybe with a warning?).

The other options you mention (`-static-libgcc` etc.) affect the link line; it looks like this patch doesn't change the way linking works?  Is that expected?  What's the normal way people use blocks on Windows?

https://github.com/llvm/llvm-project/pull/125384