[clangd] SIGSEGV at clangd: DiagnosticConsumer Is Used After Free

This is a follow-up patch for D148088. The dynamic symbol index (`FileIndex::updatePreamble`) may run in a separate thread, and the `DiagnosticConsumer` that is set up in `buildPreamble` might go out of scope before it is used. This could result in a SIGSEGV when attempting to call any method of the `DiagnosticConsumer` class.

The function `buildPreamble` sets up the `DiagnosticConsumer` as follows:
```
... buildPreamble(...) {
...
  StoreDiags PreambleDiagnostics;
  ...
  llvm::IntrusiveRefCntPtr<DiagnosticsEngine> PreambleDiagsEngine =
    CompilerInstance::createDiagnostics(&CI.getDiagnosticOpts(),
                                        &PreambleDiagnostics,
                                        /*ShouldOwnClient=*/false);
  ...
  // The call might use the diagnostic consumer in a separate thread
  PreambleCallback(...)
  ...
}
```
`PreambleDiagnostics` might be out of scope for `buildPreamble` function when we call it inside `PreambleCallback` in a separate thread.

The Fix
The fix involves replacing the client (DiagnosticConsumer) with an `IgnoringDiagConsumer` instance, which will print messages to the clangd log.

Alternatively, we can replace `PreambleDiagnostics` with an object that is owned by `DiagnosticsEngine`.

Note
There is no corresponding LIT/GTest for this issue, since there is a specific race condition that is difficult to reproduce within a test framework.

Test Plan:
```
ninja check-clangd
```

Reviewed By: kadircet, sammccall

Differential Revision: https://reviews.llvm.org/D159363