Verify mangled names against descriptions of signatures computed from IR

...instead of those computed from a frontend representation
(`symbol.signature`).

This is more robust; besides, we are going to turn off building
signatures from FIR, so `symbol.signature` is going to return null soon.

compiler/testData/ir/irText/declarations/multiplatform/k2/expectMemberInNotExpectClass.sig.kt.txt

@@ -12,7 +12,6 @@ class C2 : C1 {
 
   // CHECK JVM_IR:
   //   Mangled name: C2#f(){}kotlin.String
-  //   Mangled name for the signature: f(){}kotlin.String
   //   Public signature: /C2.f|9098388873611041001[0]
   //   Public signature debug description: f(){}kotlin.String
   // CHECK JS_IR NATIVE:
@@ -23,7 +22,6 @@ class C2 : C1 {
 
   // CHECK JVM_IR:
   //   Mangled name: C2{}p
-  //   Mangled name for the signature: {}p
   //   Public signature: /C2.p|6715504260787941082[0]
   //   Public signature debug description: {}p
   // CHECK JS_IR NATIVE:
@@ -33,7 +31,6 @@ class C2 : C1 {
   /* fake */ override val p: Int
     // CHECK JVM_IR:
     //   Mangled name: C2#<get-p>(){}kotlin.Int
-    //   Mangled name for the signature: <get-p>(){}kotlin.Int
     //   Public signature: /C2.p.<get-p>|5329635969197638839[0]
     //   Public signature debug description: <get-p>(){}kotlin.Int
     // CHECK JS_IR NATIVE: