Correct the order in which we cope with end-of-class-definition

semantics and improve our handling of default arguments. Specifically,
we follow this order:

  - As soon as the see the '}' in the class definition, the class is
  complete and we add any implicit declarations (default constructor,
  copy constructor, etc.) to the class.
  - If there are any default function arguments, parse them
  - If there were any inline member function definitions, parse them

As part of this change, we now keep track of the the fact that we've
seen unparsed default function arguments within the AST. See the new
ParmVarDecl::hasUnparsedDefaultArg member. This allows us to properly
cope with calls inside default function arguments to other functions
where we're making use of the default arguments.

Made some C++ error messages regarding failed initializations more
specific. 

llvm-svn: 61406