[clang] [clang-tools-extra] [clangd] [C++20] [Modules] Build Database Support (PR #199384)

[Matthew Asplund via cfe-commits]

mwasplund wrote:

Yes I will revert the inadvertent style changes and add tests before promoting this draft to an official release.

https://github.com/llvm/llvm-project/pull/199384