Implement DR21

A default template-argument shall not be specified in a friend template
declaration.

Interestingly, we properly handled default template arguments on friend
class members but not on just friend classes.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184882 91177308-0d34-0410-b5e6-96231b3b80d8