[JVM] Force lock object in JVM synchronized implementation into local.

This fixes a performance problem in the case where the lock object
is a capture and the monitor enter/exit happens directly on
field loads. When the locking happens on field loads instead of a
local, the JVM cannot prove that locking is balanced. That has
the consequence that the code is runs very slow (always in the
interpreter).

^KT-48367 Fixed.

libraries/stdlib/jvm/src/kotlin/util/Synchronized.kt

@@ -19,13 +19,18 @@ public actual inline fun <R> synchronized(lock: Any, block: () -> R): R {
         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
     }
 
+    // Force the lock object into a local and use that local for monitor enter/exit.
+    // This ensures that the JVM can prove that locking is balanced which is a
+    // prerequisite for using fast locking implementations. See KT-48367 for details.
+    val lockLocal = lock
+
     @Suppress("NON_PUBLIC_CALL_FROM_PUBLIC_INLINE", "INVISIBLE_MEMBER")
-    monitorEnter(lock)
+    monitorEnter(lockLocal)
     try {
         return block()
     }
     finally {
         @Suppress("NON_PUBLIC_CALL_FROM_PUBLIC_INLINE", "INVISIBLE_MEMBER")
-        monitorExit(lock)
+        monitorExit(lockLocal)
     }
 }