[IR] Dump IdSignatures and mangled names in irText tests

The reason #1 for this feature is that we want to test IdSignatures
generated for declarations. Currently, there is no (easy) way to ensure
that a change in the signature building logic doesn't cause any breaking
changes wrt klibs.

Now, most IdSignatures include hashed mangled names in them, so even if
we catch a regression where the included hash changes, there would be no
way of knowing immediately what caused it, unless we'd also have mangled
names in the expectations.

The reason #2 is to test the manglers themselves. Currently, there are
no tests for them. They heavily duplicate each other, this is already
causing issues (see KT-57427) that would be very hard to catch without
these tests.

^KT-58238 Fixed

compiler/testData/ir/irText/firProblems/coercionToUnitForNestedWhen.sig.kt.txt

@@ -0,0 +1,19 @@
+// CHECK:
+//   Mangled name: #consumeRestOfQuotedSequence@java.io.Reader(java.lang.StringBuilder;kotlin.Char){}
+//   Public signature: /consumeRestOfQuotedSequence|-9181202314719024538[0]
+fun Reader.consumeRestOfQuotedSequence(sb: StringBuilder, quote: Char): Unit
+
+// CHECK JVM_IR:
+//   Mangled name: #nextChar@java.io.Reader(){}kotlin.Char?
+//   Public signature: [ File 'coercionToUnitForNestedWhen.kt' <- /nextChar|8697099457416866690[0] ]
+private fun Reader.nextChar(): Char?
+
+// CHECK:
+//   Mangled name: {}BACKSLASH
+//   Public signature: [ File 'coercionToUnitForNestedWhen.kt' <- /BACKSLASH|9213821134505379379[0] ]
+private const val BACKSLASH: Char
+  // CHECK JVM_IR:
+  //   Mangled name: #<get-BACKSLASH>(){}kotlin.Char
+  //   Public signature: [ File 'coercionToUnitForNestedWhen.kt' <- /BACKSLASH.<get-BACKSLASH>|7174062972088453102[0] ]
+  private get
+