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.kt

@@ -0,0 +1,8 @@
+@file:JvmMultifileClass
+@file:JvmName("Test")
+package test
+
+annotation class Anno(val value: String)
+
+@Anno(constant)
+const val constant = "OK"