[FIR] Move ReturnTypeCalculator to :fir:providers module

Also make it an abstract class and turn ReturnTypeCalculatorForFullBodyResolve
  into object (because it has no state and there is no need to create
  different instances of it)

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/collectors/SimpleDiagnosticsCollector.kt

@@ -21,7 +21,7 @@ class SimpleDiagnosticsCollector(
         return CheckerRunningDiagnosticCollectorVisitor(
             MutableCheckerContext(
                 this,
-                ReturnTypeCalculatorForFullBodyResolve()
+                ReturnTypeCalculatorForFullBodyResolve
             ),
             components
         )