KT-28324 More exact startOffset for function/constructor declarations

Function/constructor declaration start offset is the start offset of the
corresponding declaration token, if available.

compiler/testData/ir/sourceRanges/declarations/secondaryConstructors.kt

@@ -0,0 +1,14 @@
+class C {
+    constructor() : super()
+
+    private
+    constructor(x: Int): super()
+
+    /**
+     * comment
+     */
+    constructor(x: String) : super()
+
+    @Suppress("UNUSED_VARIABLE")
+    constructor(x: Any): super()
+}