cmd/compile: require -lang=go1.14 for overlapping interfaces

[mdempsky]

Overlapping interfaces is a new language feature proposed for Go 1.14, so it shouldn't work with Go 1.13 code and earlier that builds using -lang=go1.13. (See also #34151.)

[gopherbot]

Change https://golang.org/cl/195678 mentions this issue: cmd/compile: require -lang=go1.14 for overlapping interfaces