go/analysis: use parser.SkipObjectResolution

Neither of these two analysis passes relies on the now-deprecated
object tracking in go/ast populated by go/parser.

Change-Id: I8ec18d1552aedf437b2b663a2698010a6faee7c9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/516575
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

go/analysis/passes/buildtag/buildtag.go

@@ -40,7 +40,7 @@ func runBuildTag(pass *analysis.Pass) (interface{}, error) {
 	}
 	for _, name := range pass.IgnoredFiles {
 		if strings.HasSuffix(name, ".go") {
-			f, err := parser.ParseFile(pass.Fset, name, nil, parser.ParseComments)
+			f, err := parser.ParseFile(pass.Fset, name, nil, parser.ParseComments|parser.SkipObjectResolution)
 			if err != nil {
 				// Not valid Go source code - not our job to diagnose, so ignore.
 				return nil, nil

go/analysis/passes/cgocall/cgocall.go

@@ -180,7 +180,7 @@ func typeCheckCgoSourceFiles(fset *token.FileSet, pkg *types.Package, files []*a
 		// If f is a cgo-generated file, Position reports
 		// the original file, honoring //line directives.
 		filename := fset.Position(raw.Pos()).Filename
-		f, err := parser.ParseFile(fset, filename, nil, parser.Mode(0))
+		f, err := parser.ParseFile(fset, filename, nil, parser.SkipObjectResolution)
 		if err != nil {
 			return nil, nil, fmt.Errorf("can't parse raw cgo file: %v", err)
 		}