[clang-tools-extra] [clangd] Replay macro definitions from preamble for clang-tidy checks (PR #202495)

[via cfe-commits]

QixiQAQ wrote:

I measured peak RSS when checking clang/lib/Sema/Sema.cpp using --check mode with --background-index=0:

main branch (reference): 1515156 KB (~1.445 GB)
this patch (copy): 1514092 KB (~1.444 GB)
difference: ~1 MB, negligible.

The memory overhead of copying MainFileMacros is not significant in practice.

https://github.com/llvm/llvm-project/pull/202495