go/types, types2: do not mutate arguments in NewChecker
CL 507975 resulted in new data races (as reported in #61212), because
the pkg argument to NewChecker was mutated.
Fix this by deferring the recording of the goVersion in pkg until type
checking is actually initiated via a call to Checker.Files.
Additionally, modify types2/check.go to bring it in sync with the
changes in go/types/check.go, and generate the new version_test.go from
the types2 file.
Also move parsing the version into checkFiles, for simplicity.
Fixes #61212
Change-Id: I15edb6c2cff3085622fe7c6a3b0dab531d27bd04
Reviewed-on: https://go-review.googlesource.com/c/go/+/508439
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Alan Donovan <adonovan@google.com>