[clang] [clang-tools-extra] [flang] [llvm] Reland 'Update llvm::Registry to work for LLVM shared library builds on windows' (#109024) (PR #112640)

[Mike Hommey via cfe-commits]

glandium wrote:

Ironically, this and later changes related to LLVM_BUILD_LLVM_DYLIB for Windows broke  LLVM_EXPORT_SYMBOLS_FOR_PLUGINS, which is a currently working way to use clang plugins on Windows, while LLVM_BUILD_LLVM_DYLIB doesn't work yet.

https://github.com/llvm/llvm-project/pull/112640