[llvm] [Support] Add clang tooling generated explicit visibility macros (PR #113097)

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

> Can we describe set of symbols to expose through a side-table instead ?

Windows requires this information at compile time really, which is why it must be annotated at the source level. The general approach is to explicitly specify the ABI for the library and indicate what functions are externally visible. That is, the default on Windows would be `-fvisibility=default -fvisibility-inlines-hidden`. This mode should be enabled by default once Windows works so that the behaviour is somewhat similar across all the platforms. This would actually help improve the other platforms as well as you end up the opportunity to perform additional optimizations.

https://github.com/llvm/llvm-project/pull/113097