[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/test-infrastructure-utils/tests/org/jetbrains/kotlin/test/utils/CustomTestDataUtils.kt

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2010-2022 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.test.utils
+
+import java.io.File
+
+private const val KT = ".kt"
+private const val KTS = ".kts"
+
+// Prefixes are chosen such that LL FIR test data cannot be mistaken for FIR test data.
+private const val FIR_PREFIX = ".fir"
+private const val LL_FIR_PREFIX = ".ll"
+
+const val CUSTOM_TEST_DATA_EXTENSION_PATTERN = "^(.+)\\.(fir|ll)\\.kts?\$"
+
+val File.isFirTestData: Boolean
+    get() = isCustomTestDataWithPrefix(FIR_PREFIX)
+
+/**
+ * @see File.llFirTestDataFile
+ */
+val File.isLLFirTestData: Boolean
+    get() = isCustomTestDataWithPrefix(LL_FIR_PREFIX)
+
+val File.isCustomTestData: Boolean
+    get() = isFirTestData || isLLFirTestData
+
+private fun File.isCustomTestDataWithPrefix(prefix: String): Boolean =
+    name.endsWith("$prefix$KT") || name.endsWith("$prefix$KTS")
+
+val File.firTestDataFile: File
+    get() = getCustomTestDataFileWithPrefix(FIR_PREFIX)
+
+/**
+ * An LL FIR test data file (`.ll.kt`) allows tailoring the expected output of a test to the LL FIR case. In very rare cases, LL FIR may
+ * legally diverge from the output of the K2 compiler, such as when the compiler's error behavior is deliberately unspecified. (For an
+ * example, see `kotlinJavaKotlinCycle.ll.kt`.)
+ */
+val File.llFirTestDataFile: File
+    get() = getCustomTestDataFileWithPrefix(LL_FIR_PREFIX)
+
+private fun File.getCustomTestDataFileWithPrefix(prefix: String): File =
+    if (isCustomTestDataWithPrefix(prefix)) this
+    else {
+        // Because `File` can be `.ll.kt` or `.fir.kt` test data, we have to go off `originalTestDataFileName`, which removes the prefix
+        // intelligently.
+        val originalName = originalTestDataFileName
+        val customName =
+            if (originalName.endsWith(KTS)) "${originalName.removeSuffix(KTS)}$prefix$KTS"
+            else "${originalName.removeSuffix(KT)}$prefix$KT"
+        parentFile.resolve(customName)
+    }
+
+val File.originalTestDataFile: File
+    get() {
+        val originalName = originalTestDataFileName
+        return if (originalName != name) parentFile.resolve(originalName) else this
+    }
+
+val File.originalTestDataFileName: String
+    get() = when {
+        isLLFirTestData -> getOriginalTestDataFileNameFromPrefix(LL_FIR_PREFIX)
+        isFirTestData -> getOriginalTestDataFileNameFromPrefix(FIR_PREFIX)
+        else -> name
+    }
+
+private fun File.getOriginalTestDataFileNameFromPrefix(prefix: String): String =
+    if (name.endsWith(KTS)) "${name.removeSuffix("$prefix$KTS")}$KTS"
+    else "${name.removeSuffix("$prefix$KT")}$KT"