[FIR2IR] Manage function symbols in declaration storage instead of declaration generator

This is needed to be able to implement creation of unbound symbols
  for references of corresponding declarations (KT-62856)

There was an exception from FIR2IR that was fixed with this change,
  so fir2ir test expectDeclarationWithWeakIncompatibilities started to
  pass along with IrActualizer, which reported some new errors

compiler/testData/diagnostics/tests/multiplatform/headerClass/expectDeclarationWithWeakIncompatibilities.fir.kt

@@ -2,14 +2,14 @@
 // MODULE: m1-common
 // FILE: common.kt
 
-expect class Foo1
-expect class Foo2
+<!EXPECT_ACTUAL_INCOMPATIBILITY{JVM}!>expect class Foo1<!>
+<!EXPECT_ACTUAL_INCOMPATIBILITY{JVM}!>expect class Foo2<!>
 
 expect fun foo2(): Int
 
-expect val s: String
+<!EXPECT_ACTUAL_INCOMPATIBILITY{JVM}!>expect val s: String<!>
 
-expect open class Foo3
+<!EXPECT_ACTUAL_INCOMPATIBILITY{JVM}!>expect open class Foo3<!>
 
 // MODULE: m2-jvm()()(m1-common)