x/tools/gopls: fillreturns modifies existing returns

[stamblerre]

I added a return value to a function, and observed that hitting save overrode the existing values with nils. (Repro in the file golang.org/x/tools/internal/lsp/source/types_format.go`.)

[gopherbot]

Change https://golang.org/cl/233527 mentions this issue: internal/lsp/source: don't mark fillreturns as high confidence

[gopherbot]

Change https://golang.org/cl/236617 mentions this issue: internal/lsp/analysis/fillreturns: broaden type equality while filling incomplete return statements