internal/lsp/cache: don't build symbol info for non-Go files

Our symbol query only searches Go files, so there is no point to
building (broken) symbol information for non-Go files. Doing so
introduces a lot more symbol handles that need to be tracked and walked.

Change-Id: I96dd62766d079805fcb1d16eb361adfc0c31eea1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/415199
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

internal/lsp/cache/snapshot.go

@@ -1010,6 +1010,10 @@ func (s *snapshot) Symbols(ctx context.Context) map[span.URI][]source.Symbol {
 		result   = make(map[span.URI][]source.Symbol)
 	)
 	s.files.Range(func(uri span.URI, f source.VersionedFileHandle) {
+		if s.View().FileKind(f) != source.Go {
+			return // workspace symbols currently supports only Go files.
+		}
+
 		// TODO(adonovan): upgrade errgroup and use group.SetLimit(nprocs).
 		iolimit <- struct{}{} // acquire token
 		group.Go(func() error {