go.tools/ssa: two fixes to CallCommon.

(*CallCommon).Signature() now returns non-nil even for
built-ins.  Builtins are now created with specialized types for
each use.  Added sanity-check.

CallCommon.HasEllipsis field eliminated.  It was an incorrect
memoization of Signature().IsVariadic() used only for
printing.

Also: introduce and use newTypeVar utility.

R=gri
CC=golang-codereviews
https://golang.org/cl/46880044
9 files changed