Implement the rules in C++ [basic.link] and C99 6.2.2 for computing

the linkage of a declaration. Switch the lame (and completely wrong)
NamedDecl::hasLinkage() over to using the new NamedDecl::getLinkage(),
along with the "can this declaration be a template argument?" check
that started all of this.

Fixes -fsyntax-only for PR5597.

llvm-svn: 89891