Load annotations of const properties from multifile classes

Rework backing field generation logic in PropertyCodegen to switch the
ClassBuilder instance for a multifile part to that of the corresponding facade
class. This became needed because multifile parts, and their metadata, are
generated _before_ the multifile facade class and otherwise we would never
record that there's a synthetic '$annotations' method for a const val and would
not write that to the protobuf message for the property.

See also bad83200

 #KT-10892 Fixed

compiler/testData/loadJava/compiledKotlinWithStdlib/annotations/ConstValInMultifileClass.txt

@@ -0,0 +1,10 @@
+package test
+
+@test.Anno(value = "OK") public const val constant: kotlin.String = "OK"
+    public fun <get-constant>(): kotlin.String
+
+public final annotation class Anno : kotlin.Annotation {
+    /*primary*/ public constructor Anno(/*0*/ value: kotlin.String)
+    public final val value: kotlin.String
+        public final fun <get-value>(): kotlin.String
+}