[clangd] Treat 'auto' params as deduced if there's a single instantiation.

This makes hover/go-to-definition/expand-auto etc work for auto params in many
common cases.
This includes when a generic lambda is passed to a function accepting
std::function. (The tests don't use this case, it requires a lot of setup).

Note that this doesn't affect the AST of the function body itself, cause its
nodes not to be dependent, improve code completion etc.
(These sort of improvements seem possible, in a similar "if there's a single
instantiation, traverse it instead of the primary template" way).

Fixes https://github.com/clangd/clangd/issues/493
Fixes https://github.com/clangd/clangd/issues/1015

Differential Revision: https://reviews.llvm.org/D119537