go/types, types2: call mustParse when using mustTypecheck

go/types, types2: call mustParse when using mustTypecheck

Syntactically incorrect source files may produce valid (but
unexpected) syntax trees, leading to difficult to understand
test failures.

Make sure to call mustParse when we call mustTypecheck.

Change-Id: I9f5ba3fe57ad3bbc16caabf285d2e7aeb5b9de0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/489995
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>