FIR LT: Introduce source file abstraction, carry it from parsing to IR

along with source lines mapping, allows to "emulate" usage of the
PSI files which allows to extract source file and line mapping info
on every stage from source element.
It makes sense to use this mapping for the error reporting too.

compiler/test-infrastructure/tests/org/jetbrains/kotlin/test/services/SourceFileProvider.kt

@@ -6,11 +6,13 @@
 package org.jetbrains.kotlin.test.services
 
 import com.intellij.openapi.project.Project
+import org.jetbrains.kotlin.KtInMemoryTextSourceFile
 import org.jetbrains.kotlin.fir.lightTree.LightTree2Fir
-import org.jetbrains.kotlin.fir.lightTree.LightTreeFile
 import org.jetbrains.kotlin.psi.KtFile
+import org.jetbrains.kotlin.sourceFiles.LightTreeFile
 import org.jetbrains.kotlin.test.model.TestFile
 import org.jetbrains.kotlin.test.util.KtTestUtil
+import org.jetbrains.kotlin.toSourceLinesMapping
 import java.io.File
 
 abstract class SourceFilePreprocessor(val testServices: TestServices) {
@@ -101,9 +103,10 @@ fun SourceFileProvider.getKtFilesForSourceFiles(testFiles: Collection<TestFile>,
 
 fun SourceFileProvider.getLightTreeKtFileForSourceFile(testFile: TestFile): LightTreeFile {
     val shortName = testFile.name.substringAfterLast('/').substringAfterLast('\\')
-    val file = getRealFileForSourceFile(testFile)
-    val lightTree = LightTree2Fir.buildLightTree(file.readText())
-    return LightTreeFile(lightTree, shortName, "/$shortName") // emulating behavior of KtTestUtil.createFile so path looks the same in testdata
+    val sourceFile = KtInMemoryTextSourceFile(shortName, "/$shortName", getContentOfSourceFile(testFile))
+    val linesMapping = sourceFile.text.toSourceLinesMapping()
+    val lightTree = LightTree2Fir.buildLightTree(sourceFile.text)
+    return LightTreeFile(lightTree, sourceFile, linesMapping)
 }
 
 fun SourceFileProvider.getLightTreeFilesForSourceFiles(testFiles: Collection<TestFile>): Map<TestFile, LightTreeFile> {