DebugInfo: Add support for template parameters with qualifiers

eg: t1<void () const> - DWARF doesn't have a particularly nice way to
encode this, for real member function types (like `void (t1::*)()
const`) the const-ness is encoded in the type of the artificial first
parameter. But `void () const` has no parameters, so encode it like a
normal const-qualified type, using DW_TAG_const_type. (similarly for
restrict and volatile)

Reference qualifiers (& and &&) coming in a separate commit shortly.