[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/cyclicHierarchy/kotlinJavaKotlinCycle.kt

@@ -1,4 +1,3 @@
-// FIR_IDE_IGNORE
 // FILE: I.kt
 
 open class I : <!CYCLIC_INHERITANCE_HIERARCHY!>K<!>() {