go.tools/cmd/gotype: make gotype more tolerant in case of 'import "C"'

[griesemer]

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.)

[minux]

Comment 1:

Status changed to Duplicate.

Merged into issue #6774.