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/classes.kt

@@ -0,0 +1,23 @@
+class Test1
+
+
+internal
+class Test2
+
+
+/**
+ * comment
+ */
+class Test3
+
+
+@Suppress("UNUSED_VAR")
+class Test4
+
+
+enum
+class Test5
+
+
+annotation
+class Test6