[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/openInInterface.fir.kt

@@ -1,10 +1,10 @@
 interface My {
     open fun foo()
     open fun bar() {}
-    open abstract fun baz(): Int
+    <!REDUNDANT_MODIFIER!>open<!> abstract fun baz(): Int
 
     open val x: Int
     open val y: String
         get() = ""
-    open abstract val z: Double
+    <!REDUNDANT_MODIFIER!>open<!> abstract val z: Double
 }