Support explicit template specialization and instantiation for members

Support explicit template specialization and instantiation for members
of a C++0x inline namespace within enclosing namespaces, as noted in
C++0x [namespace.def]p8.

Fixes <rdar://problem/9006349>, a libc++ failure where Clang was
rejected an explicit specialization of std::swap (since libc++ puts it
into an inline, versioned namespace std::__1).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127162 91177308-0d34-0410-b5e6-96231b3b80d8