go/types: make go/types more tolerant in case of 'import "C"' #6774
Description
gotype $GOROOT/src/pkg/net leads to several error messages due to unresolvable 'import "C"'. go/types could be more tolerant with invalid operands and types to avoid follow-up errors. (For instance, conversions of the form T(x) could always succeed even if x is invalid. Other operations should not report an error if an operand is already invalid.)