go/ast: add File.GoVersion

For #57001, compilers and others tools will need to understand that
a different Go version can be used in different files in a program,
according to the //go:build lines in those files.

This CL adds a GoVersion string field to ast.File, to allow exposing this
per-file Go version information.

For #59033.

Change-Id: I3931ea86c237983d152964f48dce498bcb1f06aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/476276
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>

api/next/59033.txt

@@ -1,2 +1,3 @@
+pkg go/ast, type File struct, GoVersion string #59033
 pkg go/build/constraint, func GoVersion(Expr) string #59033
 

src/go/ast/ast.go

@@ -1046,6 +1046,7 @@ type File struct {
 	Imports            []*ImportSpec   // imports in this file
 	Unresolved         []*Ident        // unresolved identifiers in this file
 	Comments           []*CommentGroup // list of all comments in the source file
+	GoVersion          string          // minimum Go version required by //go:build or // +build directives
 }
 
 // Pos returns the position of the package declaration.

src/go/ast/example_test.go

@@ -136,7 +136,8 @@ func main() {
 	//     57  .  Unresolved: []*ast.Ident (len = 1) {
 	//     58  .  .  0: *(obj @ 29)
 	//     59  .  }
-	//     60  }
+	//     60  .  GoVersion: ""
+	//     61  }
 }
 
 // This example illustrates how to remove a variable declaration

src/go/ast/filter.go

@@ -491,5 +491,5 @@ func MergePackageFiles(pkg *Package, mode MergeMode) *File {
 	}
 
 	// TODO(gri) need to compute unresolved identifiers!
-	return &File{doc, pos, NewIdent(pkg.Name), decls, minPos, maxPos, pkg.Scope, imports, nil, comments}
+	return &File{doc, pos, NewIdent(pkg.Name), decls, minPos, maxPos, pkg.Scope, imports, nil, comments, ""}
 }