FIR CLI: use VFS for LT files loading

may improve performance in some cases and is consistent with handling
of PSI files

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/VfsBasedProjectEnvironment.kt

@@ -107,6 +107,9 @@ open class VfsBasedProjectEnvironment(
         baseModuleData: FirModuleData,
         fileSearchScope: AbstractProjectFileSearchScope
     ) = FirJavaFacade(firSession, baseModuleData, project.createJavaClassFinder(fileSearchScope.asPsiSearchScope()))
+
+    override fun getFileText(filePath: String): String? =
+        localFileSystem.findFileByPath(filePath)?.inputStream?.reader(Charsets.UTF_8)?.readText()
 }
 
 private fun AbstractProjectFileSearchScope.asPsiSearchScope() =

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/pipeline/compilerPipeline.kt

@@ -311,8 +311,14 @@ fun compileModuleToAnalyzedFir(
     val countFilesAndLines = if (performanceManager == null) null else performanceManager::addSourcesStats
 
     // raw fir
-    val commonRawFir = commonSession?.buildFirViaLightTree(input.commonSources, diagnosticsReporter, countFilesAndLines)
-    val rawFir = session.buildFirViaLightTree(input.platformSources, diagnosticsReporter, countFilesAndLines)
+    val commonRawFir = commonSession?.buildFirViaLightTree(
+        input.commonSources,
+        environment.projectEnvironment,
+        diagnosticsReporter,
+        countFilesAndLines
+    )
+    val rawFir =
+        session.buildFirViaLightTree(input.platformSources, environment.projectEnvironment, diagnosticsReporter, countFilesAndLines)
 
     // resolution
     commonSession?.apply {