completions/inference: infer polymorphic types in completions

When using generic functions, the LSP now tries to infer an instantiation
based on the surroundings of the call expression. If successful, it improves
completions for parameters of generic functions. It shouldn't collide
with any pre-existing code paths.

Fixes golang/go#69754

Change-Id: Ic4552d1b988011f5b7e02d83f7938c65a23b61e0
GitHub-Last-Rev: 57eb2ac
GitHub-Pull-Request: #530
Reviewed-on: https://go-review.googlesource.com/c/tools/+/618675
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>

Author: jacobzim-stl