[FIR] Support 4 diagnostics for pairs of modifiers

In particular, this commit includes:
* Attempt to abstract access to FirSourceElement via FirModifier
* Add more visit functions to DeclarationCheckersDiagnosticComponent
* Add messages+factories for 4 modifier-related errors and warnings
* Introduce FirModifierChecker

compiler/testData/diagnostics/tests/modifiers/privateInInterface.fir.kt

@@ -1,6 +1,6 @@
 interface My {
     private val x: Int
-    private abstract val xx: Int
+    <!INCOMPATIBLE_MODIFIERS!>private<!> <!INCOMPATIBLE_MODIFIERS!>abstract<!> val xx: Int
     private val xxx: Int
         get() = 0
     final val y: Int