[FIR2IR] Unwrap call-site substitution overrides

Generating IR declarations for use-site substitution overrides leads
to IR that is different from K1 as well as problems in signature
generation which relies on mangling. Use-site substitutions can contain
references to type parameters from the call-site which aren't handled
in mangling.

#KT-57022 Fixed

compiler/testData/ir/irText/firProblems/SignatureComputationComplexJavaGeneric.fir.kt.txt

@@ -1,16 +0,0 @@
-class Test<T : Any?> : J<T> {
-  constructor(x: T) /* primary */ {
-    super/*J*/<T>()
-    /* <init>() */
-
-  }
-
-  val x: T
-    field = x
-    get
-
-  fun test(b: B<T>) {
-    b.output(x = <this>.<get-x>())
-  }
-
-}