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/primaryConstructors.kt

@@ -0,0 +1,14 @@
+class Test1(val x: Int)
+
+class Test2
+internal constructor(val x: Int)
+
+class Test3
+/**
+ * comment
+ */
+constructor(val x: Int)
+
+class Test4
+@Suppress("UNUSED_VARIABLE")
+constructor(val x: Int)