[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/fir/analysis-tests/testData/resolve/diagnostics/repeatedModifier.kt

@@ -0,0 +1,17 @@
+open <!REPEATED_MODIFIER!>open<!> class A
+
+internal <!REPEATED_MODIFIER!>internal<!> object B
+
+enum <!REPEATED_MODIFIER!>enum<!> class C {
+    VALUE1, VALUE2;
+
+    protected <!REPEATED_MODIFIER!>protected<!> companion object {
+        private <!REPEATED_MODIFIER!>private<!> val D = 5
+    }
+
+    inline <!REPEATED_MODIFIER!>inline<!> fun foo(f: (Int) -> Int) = f(8)
+}
+
+open class E(private <!REPEATED_MODIFIER!>private<!> val int: Int = 5) {
+    protected <!REPEATED_MODIFIER!>protected<!> var double = int + 8.0
+}