[ADT] Make iterable SmallVector template overrides more specific

Summary:
This prevents the iterator overrides from being selected in
the case where non-iterator types are used as arguments, which
is of particular importance in cases where other overrides with
identical types exist.

Reviewers: dblaikie, bkramer, rafael

Subscribers: llvm-commits, efriedma

Differential Revision: https://reviews.llvm.org/D33919

llvm-svn: 305105