[clangd] Fix crashing race in ClangdServer shutdown with stdlib indexing

In principle it's OK for stdlib-indexing tasks to run after the TUScheduler is
destroyed, as mostly they just update the dynamic index. We do drain the
stdlib-indexing queue before destroying the index.

However the task captures references to the PreambleCallbacks object, which is
owned by the TUScheduler. Once this is destroyed (explicitly, early in
~ClangdServer) an outstanding stdlib-indexing task may use-after-free.

The fix here is to avoid capturing references to the PreambleCallbacks.
Alternatives would be to have TUScheduler (exclusively) not own its callbacks
so they could live longer, or explicitly stopping the TUScheduler instead of
early-destroying it. These both seem more invasive.

See https://reviews.llvm.org/D115232 for some more context.

Differential Revision: https://reviews.llvm.org/D140486