gopls/internal/golang: Extract function: simplify using Cursor

This change deletes adjustRangeForCommentsAndWhiteSpace,
which duplicates functionality in PathEnclosingInterval
to turn a rough selection (including extraneous whitespace)
into a precise selection aligned with syntax nodes.

Instead we use a new function, astutil.Select, that does
the same thing but on Cursors, and push cursors throughout
the rest of the extract logic. In due course it should
replace all existing uses of PathEnclosingInterval.

Extract Variable will be dealt with in a follow-up.

Change-Id: I7c67757ffa4233d5e7a6f02e44e68e14c46faee7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/716340
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: adonovan

gopls/internal/golang/codeaction.go

@@ -483,7 +483,7 @@ func goDoc(ctx context.Context, req *codeActionsRequest) error {
 // refactorExtractFunction produces "Extract function" code actions.
 // See [extractFunction] for command implementation.
 func refactorExtractFunction(ctx context.Context, req *codeActionsRequest) error {
-	if _, ok, _, _ := canExtractFunction(req.pgf.Tok, req.start, req.end, req.pgf.Src, req.pgf.Cursor); ok {
+	if _, ok, _, _ := canExtractFunction(req.pgf.Cursor, req.start, req.end); ok {
 		req.addApplyFixAction("Extract function", fixExtractFunction, req.loc)
 	}
 	return nil
@@ -492,7 +492,7 @@ func refactorExtractFunction(ctx context.Context, req *codeActionsRequest) error
 // refactorExtractMethod produces "Extract method" code actions.
 // See [extractMethod] for command implementation.
 func refactorExtractMethod(ctx context.Context, req *codeActionsRequest) error {
-	if _, ok, methodOK, _ := canExtractFunction(req.pgf.Tok, req.start, req.end, req.pgf.Src, req.pgf.Cursor); ok && methodOK {
+	if _, ok, methodOK, _ := canExtractFunction(req.pgf.Cursor, req.start, req.end); ok && methodOK {
 		req.addApplyFixAction("Extract method", fixExtractMethod, req.loc)
 	}
 	return nil