Effectively this is a return of "${facadeName}__${partName}" scheme.

Suggestions from @udalov:
- Make multifile class parts synthetic.
- Multifile class parts should have well-formed Java identifiers as names.
- Multifile class part names should contain facade name.
Add test with clashing part names.
Add test with local generic classes used in method signatures
(it works with parts names not being well-formed Java identifiers, though).

Capitalized annotations.

compiler/testData/codegen/boxMultiFile/samePartNameDifferentFacades/1/part.kt

@@ -0,0 +1,5 @@
+@file:JvmName("Foo")
+@file:JvmMultifileClass
+package test
+
+fun foo(): String = "O"

compiler/testData/codegen/boxMultiFile/samePartNameDifferentFacades/2/part.kt

@@ -0,0 +1,5 @@
+@file:JvmName("Bar")
+@file:JvmMultifileClass
+package test
+
+fun bar(): String = "K"

compiler/testData/codegen/boxMultiFile/samePartNameDifferentFacades/box.kt

@@ -0,0 +1,3 @@
+package test
+
+fun box(): String = foo() + bar()