cmd/compile/internal/types2: simplify use of TestManual
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 <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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