go/types, types2: record instantiated type when fully instantiated function call is missing arguments

[findleyr]

Raised in CL 394017 by @muirdm, the type checker is recording the generic type for a called function if arguments are missing, even if all necessary type arguments are provided, as in the following:

func foo[A int|string](a A) {}

foo[int]() // recorded function type is still func[A int|string](A), even though it should be known to be func(int)

Example: https://go.dev/play/p/xBmccoNdCK_Y

This is probably a low priority (hence not adding the 1.19 milestone), but it would be nice to clean this up as we work on improved type inference.

CC @griesemer

[gopherbot]

Change https://go.dev/cl/400614 mentions this issue: lsp/completion: further improve generic func arg ranking

[gopherbot]

Change https://go.dev/cl/403354 mentions this issue: guru: Add a TODO list to the guru cmd.

[gopherbot]

Change https://go.dev/cl/403355 mentions this issue: passes/unusedwrites: Add TODO for how to handle generics.