[IR] In irText tests, test full mangled names only computed from IR

We plan to disable computing full mangled names of declarations in all
manglers except the IR mangler (see the subsequent commits).

From now on, in irText tests we dump only the following mangled names:

- Full mangled names computed using the IR mangler
- Signature mangled names computed using the Descriptor mangler
- Signature mangled names computed using the IR mangler
- Signature mangled names computed using the FIR mangler

Here by a full mangled name we mean the mangled name of a declaration
computed using the `MangleMode.FULL` mode. Those mangled names include
the mangled names of the declaration'a parents.

By a signature mangled name we mean the mangled name of a declaration
computed using the `MangleMode.SIGNATURE` mode.
These mangled names are used to compute an `IdSignature` for
the declaration, hence the name.

compiler/testData/ir/irText/errors/unresolvedReference.sig.kt.txt

@@ -4,8 +4,9 @@
 //   Public signature debug description: {}test1
 val test1: ErrorType
   // CHECK JVM_IR:
-  //   Mangled name computed from Ir:         #<get-test1>(){}<ERROR CLASS>
-  //   Mangled name computed from Descriptor: #<get-test1>(){}<Error class: unknown class>
+  //   Mangled name: #<get-test1>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Ir:         <get-test1>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Descriptor: <get-test1>(){}<Error class: unknown class>
   //   Public signature: /test1.<get-test1>|-4817139884021780494[0]
   //   Public signature debug description: <get-test1>(){}<Error class: unknown class>
   get
@@ -16,8 +17,9 @@ val test1: ErrorType
 //   Public signature debug description: {}test2
 val test2: ErrorType
   // CHECK JVM_IR:
-  //   Mangled name computed from Ir:         #<get-test2>(){}<ERROR CLASS>
-  //   Mangled name computed from Descriptor: #<get-test2>(){}<Error class: unknown class>
+  //   Mangled name: #<get-test2>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Ir:         <get-test2>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Descriptor: <get-test2>(){}<Error class: unknown class>
   //   Public signature: /test2.<get-test2>|1859255286326922853[0]
   //   Public signature debug description: <get-test2>(){}<Error class: unknown class>
   get
@@ -28,8 +30,9 @@ val test2: ErrorType
 //   Public signature debug description: {}test3
 val test3: ErrorType
   // CHECK JVM_IR:
-  //   Mangled name computed from Ir:         #<get-test3>(){}<ERROR CLASS>
-  //   Mangled name computed from Descriptor: #<get-test3>(){}<Error class: unknown class>
+  //   Mangled name: #<get-test3>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Ir:         <get-test3>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Descriptor: <get-test3>(){}<Error class: unknown class>
   //   Public signature: /test3.<get-test3>|-2322568342617474229[0]
   //   Public signature debug description: <get-test3>(){}<Error class: unknown class>
   get
@@ -40,8 +43,9 @@ val test3: ErrorType
 //   Public signature debug description: {}test4
 val test4: ErrorType
   // CHECK JVM_IR:
-  //   Mangled name computed from Ir:         #<get-test4>(){}<ERROR CLASS>
-  //   Mangled name computed from Descriptor: #<get-test4>(){}<Error class: unknown class>
+  //   Mangled name: #<get-test4>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Ir:         <get-test4>(){}<ERROR CLASS>
+  //   Mangled name for the signature computed from Descriptor: <get-test4>(){}<Error class: unknown class>
   //   Public signature: /test4.<get-test4>|-5181690683324812065[0]
   //   Public signature debug description: <get-test4>(){}<Error class: unknown class>
   get