cmd/compile: don't apply -lang=go1.X restrictions to imported packages

Previously langSupported applied -lang as though it's a global
restriction, but it's actually a per-package restriction. This CL
fixes langSupported to take a *types.Pkg parameter to reflect this and
updates its callers accordingly.

This is relevant for signed shifts (added in Go 1.12), because they
can be inlined into a Go 1.11 package; and for overlapping interfaces
(added in Go 1.13), because they can be exported as part of the
package's API.

Today we require all Go packages to be compiled with the same
toolchain, and all uses of langSupported are for controlling
backwards-compatible features. So we can simply assume that since the
imported packages type-checked successfully, they must have been
compiled with an appropriate -lang setting.

In the future if we ever want to use langSupported to control
backwards-incompatible language changes, we might need to record the
-lang flag used for compiling a package in its export data.

Fixes #35437.
Fixes #35442.

Change-Id: Ifdf6a62ee80cd5fb4366cbf12933152506d1b36e
Reviewed-on: https://go-review.googlesource.com/c/go/+/205977
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -1477,8 +1477,18 @@ type lang struct {
 // any language version is supported.
 var langWant lang
 
-// langSupported reports whether language version major.minor is supported.
-func langSupported(major, minor int) bool {
+// langSupported reports whether language version major.minor is
+// supported in a particular package.
+func langSupported(major, minor int, pkg *types.Pkg) bool {
+	if pkg == nil {
+		// TODO(mdempsky): Set Pkg for local types earlier.
+		pkg = localpkg
+	}
+	if pkg != localpkg {
+		// Assume imported packages passed type-checking.
+		return true
+	}
+
 	if langWant.major == 0 && langWant.minor == 0 {
 		return true
 	}