cmd/compile/internal/types2: simplify use of TestManual

Running the TestManual test (for manual debugging) requires
user-provided files as input. Rather than using another flag
(-files) to provide these files, just use the (remaining)
command line arguments.

Change-Id: I9b20d9f1a6a7ce839bbd690c311ce3f0d0a10496
Reviewed-on: https://go-review.googlesource.com/c/go/+/315689


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>