[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/incompatibleVarianceModifiers.fir.kt

@@ -1,8 +1,8 @@
 // !DIAGNOSTICS: -UNUSED_VARIABLE
 
 interface Foo<T>
-interface Foo1<in out T>
-interface Foo2<in in T>
+interface Foo1<<!INCOMPATIBLE_MODIFIERS!>in<!> <!INCOMPATIBLE_MODIFIERS!>out<!> T>
+interface Foo2<in <!REPEATED_MODIFIER!>in<!> T>
 
 fun test1(foo: Foo<in out Int>) = foo
 fun test2(): Foo<in in Int> = throw Exception()
@@ -10,11 +10,11 @@ fun test2(): Foo<in in Int> = throw Exception()
 fun test3() {
     val f: Foo<out out out out Int>
 
-    class Bzz<in in T>
+    class Bzz<in <!REPEATED_MODIFIER!>in<!> T>
 }
 
 class A {
-    fun <out out T> bar() {
+    fun <out <!REPEATED_MODIFIER!>out<!> T> bar() {
     }
 }