syncs: add means of declare locking assumptions for debug mode validation

Updates #17852

Change-Id: I42a64a990dcc8f708fa23a516a40731a19967aba
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

syncs/mutex.go

@@ -16,3 +16,8 @@ type Mutex = sync.Mutex
 //
 // It's only not a sync.RWMutex when built with the ts_mutex_debug build tag.
 type RWMutex = sync.RWMutex
+
+// RequiresMutex declares the caller assumes it has the given
+// mutex held. In non-debug builds, it's a no-op and compiles to
+// nothing.
+func RequiresMutex(mu *sync.Mutex) {}