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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

Just to confirm I understand the broader context: we only need it when building the AST because the rest of the code (completions, signature help) actually already uses a compile command and FS from preamble inputs, right

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