[clangd] Smarter hover on auto and decltype

[clangd] Smarter hover on auto and decltype

Only show the keyword as the hover "Name".

Show whether the type is deduced or undeduced as
the hover "Documentation".

Show the deduced type (if any) as the "Definition".

Don't show any hover information for:
- the "auto" word of "decltype(auto)"
- "auto" in lambda parameters
- "auto" in template arguments

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This diff is a suggestion based on what @sammccall  suggested in https://reviews.llvm.org/D92977 about hover on "auto". It somehow "hacks" onto the "Documentation" and "Definition" fields of `HoverInfo`. It sure looks good on VSCode, let me know if this seem acceptable to you.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D93227