IR: .isBound API for F/O declarations

Fake override functions and properties may not be bound to symbols.
This API allows to know if such declaration is actually bound

compiler/ir/ir.tree.impl/src/org/jetbrains/kotlin/ir/declarations/impl/IrFunctionImpl.kt

@@ -139,4 +139,7 @@ class IrFakeOverrideFunctionImpl(
         symbol.bind(this)
         return this
     }
+
+    override val isBound: Boolean
+        get() = _symbol != null
 }

compiler/ir/ir.tree.impl/src/org/jetbrains/kotlin/ir/declarations/impl/IrPropertyImpl.kt

@@ -126,4 +126,7 @@ class IrFakeOverridePropertyImpl(
         symbol.bind(this)
         return this
     }
+
+    override val isBound: Boolean
+        get() = _symbol != null
 }