SmallVector's growth policies don't like starting from zero capacity.

SmallVector's growth policies don't like starting from zero capacity.
I think there are good reasons to change this, but in the interests
of short-term stability, make SmallVector<...,0> reserve non-zero
capacity in its constructors.  This means that SmallVector<...,0>
uses more memory than SmallVector<...,1> and should really only be
used (unless/until this workaround is removed) by clients that
care about using SmallVector with an incomplete type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112147 91177308-0d34-0410-b5e6-96231b3b80d8