Implement SYCL bindless sampled image builtin with ImageSampleExplicitLod

Change from __spirv_ImageRead to __spirv_ImageSampleExplicitLod in
SYCL bindless sampled image builtins since in SPIR-V spec ImageRead only
applies to unsampled image.
In SPV-IR after llvm-spirv translation, OpenCL sampled image builtin is
also represented as __spirv_ImageSampleExplicitLod.
SYCL header will be updated later to use __spirv_ImageSampleExplicitLod
for sampled image.

Author: wenju-he