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

[Thomas Fransham via llvm-commits]

fsfod wrote:

The whole idea is people won't have to have to manually add or maintain them, it will be automated with a clang tooling based tool that might run like gn syncbot does.

> Have you considered any alternatives ? Is it possible to have a finer granular built for windows? Can we describe set of symbols to expose through a side-table instead ?

What you describe is close to the existing non LLVM_BUILD_LLVM_DYLIB DLL build mode for windows which has the major limitation that data symbols can't be imported by code consuming the shared library.

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