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/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/context/CheckerContext.kt

@@ -87,7 +87,7 @@ abstract class CheckerContext : MutableDiagnosticContext() {
         get() = session.languageVersionSettings
 
     override val containingFilePath: String?
-        get() = containingDeclarations.firstOrNull()?.let { (it as? FirFile)?.path }
+        get() = containingDeclarations.firstOrNull()?.let { (it as? FirFile)?.sourceFile?.path }
 }
 
 /**