gopls/internal/lsp/filecache: limit Get parallelism too

This change causes Get to be subject to the same
concurrency-limiting semaphore just added to Set, as a user
(predictably) encountered thread exhaustion
in this operation too.

Oddly, the Get benchmark doesn't seem to be limited
by the value of 128, or even smaller values.

Updates golang/go#60089

Change-Id: Ie4632cbc4cdd6536558b2067a3d115cf4fa17fae
Reviewed-on: https://go-review.googlesource.com/c/tools/+/495055
Run-TryBot: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/internal/lsp/filecache/filecache.go

@@ -56,6 +56,9 @@ func Start() {
 // possibly by another process.
 // Get returns ErrNotFound if the value was not found.
 func Get(kind string, key [32]byte) ([]byte, error) {
+	iolimit <- struct{}{}        // acquire a token
+	defer func() { <-iolimit }() // release a token
+
 	name, err := filename(kind, key)
 	if err != nil {
 		return nil, err