[clang] [Clang] Add explicit visibility symbol macros (PR #108276)

[Saleem Abdulrasool via cfe-commits]

compnerd wrote:

> One thing I don't understand about this PR is why we need Compiler.h in Clang -- wouldn't the LLVM definitions in their Compiler.h work for Clang as well? (This would probably be worth explaining in the patch summary.)

The symbol lookup in PE/COFF is two level and symmetric, bound to the module. As a result, each module needs to explicitly specify the ABI interface (contract). However, given that the interface is annotated differently (`__declspec(dllexport)` for the producer, `__declspec(dllimport)` for the consumer), we need to flip the definition. When doing so, we need to know which module is being built. We cannot just use the conjunction of the module identifier to select between the two variants as there may be interdependencies with dynamic linking (e.g. `#if defined(A_ABI) || defined(B_ABI)` doesn't work if B depends on A and dynamically links to A).

I do agree that we could put this in the commit message, though, this is likely something that deserves a note in the developer documentation because I fear that other developers are going to have this question repeatedly once this work is completed. The biggest point of contention with this work was the fact that it involves an ongoing maintenance cost as the ABI boundary is now being more rigidly defined which is/was not needed to support it on Unix due to the (global) single level binding.

https://github.com/llvm/llvm-project/pull/108276