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

[kadir çetinkaya via cfe-commits]

kadircet wrote:

> 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?

That was my remembrance, but apparently it was wrong :/ Changed the logic to cover those cases as well && added some tests.

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