[OpenCL] Allow OpenCL C style vector initialization in C++

Allow creating vector literals from other vectors.

 float4 a = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
 float4 v = (float4)(a.s23, a.s01);

Differential revision: https://reviews.llvm.org/D65286

llvm-svn: 367675