ASTContext::getVectorType should preserve the vector kind for canonical types.

This is needed for Neon types when it is most natural to define them in terms
of a typedef.  For example, Neon poly8_t is a typedef for "signed char", and
we want to define polynomial vectors as vectors of that typedef.  Without this
change, the result will be a generic GCC-style vector.  I think this is safe
for other vector types as well, but I would appreciate a review of this.

llvm-svn: 119300