Introduce experimental -Xuse-javac compilation mode

In this mode, javac AST and Symbol files are used during
Kotlin compilation instead of PSI / binary stuff.
Later, they are reused for Java file compilation.
javac in this mode is integrated into kotlinc.

compiler/testData/compileKotlinAgainstJava/ReturnInnerClass.java

@@ -0,0 +1,17 @@
+package test;
+
+class ReturnInnerClass extends ReturnInnerClassImpl {
+
+}
+
+class ReturnInnerClassImpl extends AbstractReturnInnerClass {
+
+}
+
+abstract class AbstractReturnInnerClass {
+
+    class InnerClass {}
+
+    InnerClass getInnerClass() { return null; }
+
+}