[OpenCL] Set calling convention for -fdeclare-opencl-builtins

IR produced using TableGen builtin function declarations
(`fdeclare-opencl-builtins.cl`) did not have the target's calling
convention applied to builtin calls.

Fix this, and update the codegen test to check that IR produced using
opencl-c.h and `-fdeclare-opencl-builtins` is identical with respect
to the builtin calls.

Differential Revision: https://reviews.llvm.org/D98039