Fix AssertionError on property generation in multi-file classes

See 4a533168d9 for the original change which
introduced the problem

Note that the added test case _was not failing_ before the change. It's added
because there were no tests on multi-file class behavior in light classes mode
at all. The actual repro for the problem is difficult to make a test from

 #KT-12755 Fixed

compiler/testData/asJava/lightClasses/facades/MultiFile.kt

@@ -0,0 +1,8 @@
+// test.MultiFile
+
+@file:JvmMultifileClass
+@file:JvmName("MultiFile")
+
+package test
+
+val foo = 42