Remove an unsafe member variable that wasn't needed; NFC.

People use the AST dumping interface while debugging, so it's not safe to assume that a declaration will be dumped before a constant expression is dumped. This means the Context member may not get set properly and problems would happen. Rather than rely on the interface that requires the ASTContext, call the generic dump() interface instead; this allows us to remove the Context member variable.

llvm-svn: 372323