[clangd] Use TargetOpts from preamble when building ASTs (#88381)

Building ASTs with compile flags that are incompatible to the ones used
for the Preamble are not really supported by clang and can trigger
crashes.

In an ideal world, we should be re-using not only TargetOpts, but the
full ParseInputs from the Preamble to prevent such failures.

Unfortunately current contracts of ThreadSafeFS makes this a non-safe
change for certain implementations. As there are no guarantees that the
same ThreadSafeFS is going to be valid in the Context::current() we're
building the AST in.