[clangd] Add inlay hints for auto-typed parameters with one instantiation.

This takes a similar approach as b9b69381, and shares some code.
The code sharing is limited as inlay hints wants to deduce the type of the
variable rather than the type of the `auto` per-se.

It drops support (in both places) for multiple instantiations yielding the same
type, as this is pretty rare and hard to build a nice API around.

Differential Revision: https://reviews.llvm.org/D120258