Add element-type to the Vector TypeLoc types.

As shown by bug 48540, GCC vector types would cause a crash when the
declaration hada ParenType. This was because the walking of the
declaration would try to expand the 'inner' type, but there was no
ability to get it from the vector type.  This patch adds that element
type access to the vector type loc objects.

Differential Revision: https://reviews.llvm.org/D93483