Fix destructor definition of invalid classes

The declaration of the destructor of an invalid class was not properly marked
as noexcept. As a result, the definition of the same destructor, which was
properly implicitly marked as noexcept, would not match the definition.
This would cause the definition CXXDestructorDecl to be matked as invalid
and omited from the AST.

Differential Revision: http://reviews.llvm.org/D17988

llvm-svn: 263639