[clangd] Implement hover for "auto" and "decltype"

Summary:
This allows hovering on keywords that refer to deduced types.
This should cover most useful cases. Not covered:
- auto template parameters: Since this can be instantiated with many types,
it would not be practical to show the types.
- Structured binding: This could be done later to show multiple deduced types
in the hover.
- auto:: (part of concepts): Outside the scope of this patch.

Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48159

llvm-svn: 336119