[IrFakeOverrideBuilder] Do not compute signatures in Fir2IR

No one need them, while it requires significant time usage

^KT-66281
^KT-66441 Fixed

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)
 

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

@@ -0,0 +1,27 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+// MODULE: m1-common
+// FILE: common.kt
+
+expect class Foo1
+expect class Foo2
+
+expect fun foo2(): Int
+
+expect val s: String
+
+expect open class Foo3
+
+// MODULE: m2-jvm()()(m1-common)
+
+// FILE: jvm.kt
+
+interface <!ACTUAL_MISSING!>Foo1<!>
+actual interface <!ACTUAL_WITHOUT_EXPECT!>Foo2<!>
+
+actual var <!ACTUAL_WITHOUT_EXPECT!>s<!>: String = "value"
+
+fun <!ACTUAL_MISSING!>foo2<!>(): Int = 0
+
+actual class <!ACTUAL_WITHOUT_EXPECT, PACKAGE_OR_CLASSIFIER_REDECLARATION!>Foo3<!>
+
+class <!ACTUAL_MISSING, PACKAGE_OR_CLASSIFIER_REDECLARATION!>Foo3<!>