Introduce ContextDecl, patch by Argiris Kirtzidis!

-Added ContextDecl (no TranslationUnitDecl)
-ScopedDecl class has a ContextDecl member
-FieldDecl class has a ContextDecl member, so that a Field or a ObjCIvar can be traced back to their RecordDecl/ObjCInterfaceDecl easily
-FunctionDecl, ObjCMethodDecl, TagDecl, ObjCInterfaceDecl inherit from ContextDecl. With TagDecl as ContextDecl, enum constants have a EnumDecl as their context.
-Moved Decl class to a "DeclBase.h" along with ContextDecl class
-CurContext is handled by Sema

llvm-svn: 49208