K2: minor, remove misleading FirMemberDeclaration.containerSource

"Container source" means "the source of the container". So, if you
called this extension on a class (`FirClassLikeDeclaration`), it was
reasonable to assume that you would get the source of its _containing
class_, whereas the function actually returned the source of the class
itself.

Instead of inventing another name for this function, I've decided to
inline and remove it because it has only one usage.

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/utils/declarationAttributes.kt

@@ -49,12 +49,6 @@ var <T> T.danglingTypeConstraints: List<DanglingTypeConstraint>?
 
 // ----------------------------------- Utils -----------------------------------
 
-val FirMemberDeclaration.containerSource: SourceElement?
-    get() = when (this) {
-        is FirCallableDeclaration -> containerSource
-        is FirClassLikeDeclaration -> sourceElement
-    }
-
 val FirProperty.hasExplicitBackingField: Boolean
     get() = backingField != null && backingField !is FirDefaultPropertyBackingField
 

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/descriptors/utils.kt

@@ -7,8 +7,10 @@ package org.jetbrains.kotlin.backend.konan.descriptors
 
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.descriptors.impl.ModuleDescriptorImpl
+import org.jetbrains.kotlin.fir.declarations.FirCallableDeclaration
+import org.jetbrains.kotlin.fir.declarations.FirClassLikeDeclaration
 import org.jetbrains.kotlin.fir.declarations.FirMemberDeclaration
-import org.jetbrains.kotlin.fir.declarations.utils.containerSource
+import org.jetbrains.kotlin.fir.declarations.utils.sourceElement
 import org.jetbrains.kotlin.fir.lazy.AbstractFir2IrLazyDeclaration
 import org.jetbrains.kotlin.ir.declarations.IrDeclaration
 import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
@@ -62,8 +64,11 @@ private fun DeclarationDescriptor.isFromFirDeserializedInteropLibrary(): Boolean
     //  - K2 metadata deserializer doesn't set containerSource for property accessors.
     val topLevelDeclaration = declaration.findTopLevelDeclaration().propertyIfAccessor()
 
-    val firDeclaration = (topLevelDeclaration as? AbstractFir2IrLazyDeclaration<*>)?.fir ?: return false
-    val containerSource = (firDeclaration as? FirMemberDeclaration)?.containerSource
+    val firDeclaration = (topLevelDeclaration as? AbstractFir2IrLazyDeclaration<*>)?.fir as? FirMemberDeclaration ?: return false
+    val containerSource = when (firDeclaration) {
+        is FirCallableDeclaration -> firDeclaration.containerSource
+        is FirClassLikeDeclaration -> firDeclaration.sourceElement
+    }
 
     return containerSource is KlibDeserializedContainerSource && containerSource.isFromNativeInteropLibrary
 }