Unified obtaining psi element for descriptor. Now code for finding effective descriptors of fake override and finding source element for original declarations are separated. Old method DescriptorToSourceUtils.descriptorToDeclaration is still there, because it has ~90 usages.

idea/testData/quickfix/changeSignature/beforeNotEditable.kt

@@ -0,0 +1,12 @@
+// "class org.jetbrains.kotlin.idea.quickfix.RemoveFunctionParametersFix" "false"
+//ERROR: No value passed for parameter other
+
+trait StringComparable {
+    public fun compareTo(other: String): Int = 0
+}
+
+class X: Comparable<String>, StringComparable
+
+fun main(args: Array<String>) {
+    X().compareTo(<caret>)
+}