gopls/internal/golang: close open file

Found while thinking about typestate.

Change-Id: Ia98ace4782be34d1201f0ce5c8e536314fc494d7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/563155
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

gopls/internal/golang/gc_annotations.go

@@ -34,6 +34,7 @@ func GCOptimizationDetails(ctx context.Context, snapshot *cache.Snapshot, mp *me
 	if err != nil {
 		return nil, err
 	}
+	tmpFile.Close() // ignore error
 	defer os.Remove(tmpFile.Name())
 
 	outDirURI := protocol.URIFromPath(outDir)