Introduce InlineClasses language feature

Allow to write `inline` modifier in front of class declaration

compiler/testData/diagnostics/tests/inlineClasses/basicInlineClassDeclaration.kt

@@ -0,0 +1,8 @@
+// !LANGUAGE: +InlineClasses
+
+inline class Foo(val x: Int)
+
+<!WRONG_MODIFIER_TARGET!>inline<!> interface InlineInterface
+<!WRONG_MODIFIER_TARGET!>inline<!> annotation class InlineAnn
+<!WRONG_MODIFIER_TARGET!>inline<!> object InlineObject
+<!WRONG_MODIFIER_TARGET!>inline<!> enum class InlineEnum