[clang-tools-extra] [clangd] Use TargetOpts from preamble when building ASTs (PR #88381)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

All good from my side, but asked @sam-mccall or @usx95 to check the context propagation bit as I'm missing context to make sure it's correct.

https://github.com/llvm/llvm-project/pull/88381