Improve the internal representation and semantic analysis of friend

function templates.

This commit ensures that friend function templates are constructed as
FunctionTemplateDecls rather than partial FunctionDecls (as they
previously were). It then implements template instantiation for friend
function templates, injecting the friend function template only when
no previous declaration exists at the time of instantiation. 

Oh, and make sure that explicit specialization declarations are not
friends.

llvm-svn: 83970