go/types, types2: allow (string...) signature with NewSignatureType
Includes cases where the core type of the variadic parameter is
a slice or bytestring. Permits a client to create the signature
for various instantiations of append.
Fixes #55030.
Change-Id: I0f4983eb00c088cbe1d87954ee0b2df0ccc3bc49
Reviewed-on: https://go-review.googlesource.com/c/go/+/430455
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