MSVC compat: Allow lookup of friend types in enclosing namespaces

The relevant portion of C++ standard says [namespace.memdef]p3:

  If the name in a friend declaration is neither qualified nor a
  template-id and the declaration is a function or an
  elaborated-type-specifier, the lookup to determine whether the entity
  has been previously declared shall not consider any scopes outside the
  innermost enclosing namespace.

MSVC does not implement that rule for types.  If there is a type in an
enclosing namespace, they consider an unqualified tag declaration with
the same name to be a redeclaration of the type from another namespace.

Implementing compatibility is a simple matter of disabling our
implementation of this rule for types, which was added in r177473.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D4443