[OpenCL] Fix checking of vector type casting

Currently clang allows the following code

int a;
int b = (const int) a;
However it does not the following code

int4 a;
int4 b = (const int4) a;
This is because Clang compares the qualified types instead of unqualified types for vector type casting, which causes the inconsistency.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D38463

llvm-svn: 314802