[ExtVectorType] Support conditional select operator for C++.

This patch implements the conditional select operator for
ext_vector_types in C++. It does so by using the same semantics as for
C.

D71463 added support for the conditional select operator for VectorType
in C++. Unfortunately the semantics between ext_vector_type in C are
different to VectorType in C++. Select for ext_vector_type is based on
the MSB of the condition vector, whereas for VectorType it is `!= 0`.

This unfortunately means that the behavior is inconsistent between
ExtVectorType and VectorType, but I think using the C semantics for
ExtVectorType in C++ as well should be less surprising for users.

Reviewed By: erichkeane, aaron.ballman

Differential Revision: https://reviews.llvm.org/D98055