go/types: fix data race in Alias.cleanup for nil-checker instantiation

[chabbimilind]

When types.Instantiate is called with check == nil (as done by
golang.org/x/tools/go/ssa), the resulting *Alias instances had
a.actual == nil. Concurrent calls to types.Identical would then
race on the lazy initialization in unalias().

This change adds an else clause to (*Checker).newAlias to call
a.cleanup() inline when check == nil, ensuring a.actual is set
before the alias escapes.

The new test TestInstantiateAliasRace reproduces the race when
the fix is not present and passes with the fix applied.

Fixes #79035

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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