[clangd] Support configuration of inlay hints.

The idea is that the feature will always be advertised at the LSP level, but
depending on config we'll return partial or no responses.

We try to avoid doing the analysis for hints we're not going to return.

Examples of syntax:
```
InlayHints:
  Enabled: No
---
InlayHints:
  ParameterNames: No
---
InlayHints:
  ParameterNames: Yes
  DeducedTypes: Yes
```

Differential Revision: https://reviews.llvm.org/D116713