[clang-tools-extra] [clangd] Support .clangd command line modifications for C++ modules (PR #122606)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Given @kadircet is approved and @petr-polezhaev is marked as `first-time contributor`, I'll merge this by assuming you don't have the commit access.

https://github.com/llvm/llvm-project/pull/122606