Improve handling of friend types in several ways:

  - When instantiating a friend type template, perform semantic
  analysis on the resulting type.
  - Downgrade the errors concerning friend type declarations that do
  not refer to classes to ExtWarns in C++98/03. C++0x allows
  practically any type to be befriended, and ignores the friend
  declaration if the type is not a class.

llvm-svn: 100635