[LL FIR] KT-50732 Add support for LL FIR-specific tests (.ll.kt)

- `.ll.kt` test data can be added in cases where LL FIR resolution
  legally diverges from K2 compiler results.
- Each `.ll.kt` test is prefixed with an `LL_FIR_DIVERGENCE` directive
  which must explain why the test may diverge from K2 compiler results.
  - `LLFirDivergenceCommentChecker` ensures that each `.ll.kt` file
    contains an `LL_FIR_DIVERGENCE` directive.
- `LLFirIdenticalChecker` results in an assertion error if the `.ll.kt`
  test and its base test are completely identical, including in their
  meta info (but ignoring `LL_FIR_DIVERGENCE`).
  - The checker additionally ensures that the base source file and the
    `.ll.kt` source file have identical Kotlin source code (ignoring
    meta info and `LL_FIR_DIVERGENCE`). This ensures that both tests
    test the exact same thing.
- `.ll.kt` files are ignored by select test generators, in addition to
  `.fir.kt` files.

compiler/testData/diagnostics/tests/redeclarations/ClassRedeclarationInDifferentFiles.ll.kt

@@ -0,0 +1,17 @@
+// LL_FIR_DIVERGENCE
+// The compiler reports `PACKAGE_OR_CLASSIFIER_REDECLARATION` on both `A`s, but the LL FIR APIs output is also workable. The underlying
+// issue is already reported: KTIJ-23371.
+// LL_FIR_DIVERGENCE
+
+// FIR_IDENTICAL
+// FILE: f1.kt
+package test
+
+class A
+class F1
+
+// FILE: f2.kt
+package test
+
+class <!PACKAGE_OR_CLASSIFIER_REDECLARATION!>A<!>
+class F2