Initial implementation of argument dependent lookup (a.k.a. ADL,

a.k.a. Koenig lookup) in C++. Most of the pieces are in place, but for
two:

  - In an unqualified call g(x), even if the name does not refer to
    anything in the current scope, we can still find functions named
    "g" based on ADL. We don't yet have this ability.
  - ADL will need updating for friend functions and templates.

llvm-svn: 63692