go/printer: never print a newline before the returned results

Otherwise, if one ends up with a "return result" where the two nodes are
in separate lines, the printer would incorrectly print a naked return:

	return
		result

The fix is simple - by not telling exprList what the previous position
is, it never adds a leading linebreak. This is the same mechanism used
for identifier lists and values, so it seems appropriate.

All other exprList calls that can produce a leading linebreak don't seem
buggy, because closing tokens such as parentheses and colons are needed
to finish the statement.

Verified that the test failed before the patch as well:

	--- FAIL: TestIssue32854 (0.00s)
	    printer_test.go:806: got "return\n\tcall()", want "return call()"

Finally, verified that 'gofmt -l -w src misc' doesn't make any new
changes, just in case we introduced any regression.

Fixes #32854.

Change-Id: I3384fbd711de06e742407df874c9ad85626d5d6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/184121


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>