Improve parsing and instantiation of destructor names, so that we can

now cope with the destruction of types named as dependent templates,
e.g.,

  y->template Y<T>::~Y()

Nominally, we implement C++0x [basic.lookup.qual]p6. However, we don't
follow the letter of the standard here because that would fail to
parse

  template<typename T, typename U>
  X0<T, U>::~X0() { }

properly. The problem is captured in core issue 339, which gives some
(but not enough!) guidance. I expect to revisit this code when the
resolution of 339 is clear, and/or we start capturing better source
information for DeclarationNames.

Fixes PR6152.

llvm-svn: 96367