[dev.typeparams] merge dev.regabi (618e3c15) into dev.typeparams

This involved a couple non-trivial fixes in go/types:
 - move the check for main function signature to resolver.go, to be
   consistent with init. Also, update uses of _InvalidInitSig to
   _InvalidInitDecl, consistent with what we decided for dev.regabi.
 - Update some tests in api_test.go which newly fail after CL 289715
   (fixing reporting of untyped nil) In all cases but one, these updates
   were consistent with types2. However, in one case types2 seems to be
   able to resolve more type information than go/types for a broken
   package. I left a TODO to investigate this further.

Change-Id: I8244b7c81654194edd5af8de689a13c262117dff