PR15730/PR16986 Allow dependently typed vector_size types.

As listed in the above PRs, vector_size doesn't allow
dependent types/values. This patch introduces a new
DependentVectorType to handle a VectorType that has a dependent
size or type.

In the future, ALL the vector-types should be able to create one
of these to handle dependent types/sizes as well. For example,
DependentSizedExtVectorType could likely be switched to just use
this instead, though that is left as an exercise for the future.


Differential Revision: https://reviews.llvm.org/D49045

llvm-svn: 337036