[SmallVector] Reinstate the typedefs.

[SmallVector] Reinstate the typedefs.

They're unused with recent versions of libstdc++ but older ones
(e.g. libstdc++ 4.9 still requires them). Maybe we should bump
the requirements on the minimum version to make GCC 7 happy, but
in the meanwhile we need to live with the warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305158 91177308-0d34-0410-b5e6-96231b3b80d8