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/classFuns.kt

@@ -0,0 +1,24 @@
+package test
+
+
+class Test {
+
+    fun test0() {}
+
+
+    /**
+     * block comment
+     */
+    fun test1() {}
+
+
+    @Suppress(
+        "UNUSED_VARIABLE"
+    )
+    fun test2() {}
+
+
+    private
+    fun test3() {}
+
+}