Omnibus friend decl refactoring.  Instead of cloning AST classes for friend
declarations of same, introduce a single AST class and add appropriate bits
(encoded in the namespace) for whether a decl is "real" or not.  Much hackery
about previously-declared / not-previously-declared, but it's essentially
mandated by the standard that friends alter lookup, and this is at least
fairly non-intrusive.

Refactor the Sema methods specific to friends for cleaner flow and less nesting.

Incidentally solve a few bugs, but I remain confident that we can put them back.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@80353 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed