[FIR JS] Support JS_NAME_CLASH and JS_FAKE_NAME_CLASH diagnostics

^KT-59425 Fixed
^KT-59370 Fixed

compiler/testData/diagnostics/testsWithJsStdLib/name/methodAndMethod.fir.kt

@@ -1,3 +1,6 @@
+// FIR_DIFFERENCE
+// This case is only relevant for the JS Legacy BE and is not applicable to the JS IR backend,
+// as the IR BE can resolve such name collisions. Furthermore, the IR BE mangles names differently.
 package foo
 
 fun bar(x: Int) = x