x/tools/gopls: go/parser work-arounds

[stamblerre]

This issue tracks all of the workarounds implemented in gopls to handle places where go/parser loses information. Most of this code can be found in golang.org/x/tools/internal/lsp/cache/parse.go and was written by @muirdm.

The current list of workarounds includes handling:

• Missing curly braces in if statements (if foo -> if foo {})
• Missing selectors (x. -> x._)
• Partial identifiers that are keywords (foo.var -> foo._)
• Missing conditional statements in if statements with initialization statements (if i := 0)
• Unexpected array types ([]int -> []int{})
• Incomplete go or defer statements (go foo -> go foo()).
• Dangling selectors in go or defer statements

go foo.
y := 1

becomes

go foo._
y := 1