[clang] [CIR] Upstream global variable linkage types (PR #129072)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

Can you also update `clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp` to use the linkage? There are two places marked by `cir::MissingFeatures::opGlobalLinkage()` that would need to be updated, plus (hopefully) test changes.

It would also be good to have tests for more linkage types. I'm not sure there is a single existing test that verifies these. I found some in `clang/test/CIR/CodeGen/weak.c` in the incubator and some other examples in `clang/test/CodeGen/global-decls.c`. Perhaps a new `global-linkage.cpp` test could be added that cover all the variations we currently support?

https://github.com/llvm/llvm-project/pull/129072