cmd/compile: do not assume typos are new(expr) uses #75986
Description
% cat /tmp/x.go
package p
type MyType int
func f() {
_ = new(myType)
_ = new(myOtherType)
}
% go tool compile -lang=go1.25 /tmp/x.go
/tmp/x.go:6:6: new(expr) requires go1.26 or later (-lang was set to go1.25; check go.mod)
/tmp/x.go:6:10: undefined: myType
/tmp/x.go:7:6: new(expr) requires go1.26 or later (-lang was set to go1.25; check go.mod)
/tmp/x.go:7:10: undefined: myOtherType
%
The "new(expr)" errors are misleading and inaccurate and should not be emitted unless the argument typechecked successfully and is actually an expression. It looks like the current rule is "is not a valid type", which is too broad.
If you do new(strings.BUILDER) then you get
/tmp/x.go:8:6: new(expr) requires go1.26 or later (-lang was set to go1.25; check go.mod)
/tmp/x.go:8:18: undefined: strings.BUILDER (but have Builder)
which is also bad.