[SYCL] Rename __clc_native_exp2/tanh to __spirv_ocl_native_* in SYCL headers

[wenju-he]

Why:

• _clc* names are libclc-internal conventions and are confusing in SYCL headers.
• After this change, clc-remangler don't need to handle _clc* symbols.

Note: __spirv_ocl_native_tanh is not yet listed in the OpenCL Extended Instruction Set spec.

[bader]

Note: __spirv_ocl_native_tanh is not yet listed in the OpenCL Extended Instruction Set spec.

@wenju-he, how is __spirv_ocl_native_tanh translated into SPIR-V: as a function call or as an undocumented SPIR-V instruction?

@vmaksimo, @YuriPlyakhin, @bashbaug, FYI. Do we need to document this function in a SPIR-V extension?

[wenju-he]

how is __spirv_ocl_native_tanh translated into SPIR-V: as a function call or as an undocumented SPIR-V instruction?

It is a function call in SPIR-V if we enable it for SPIR-V path:

     %call_i = OpFunctionCall %float %_Z23__spirv_ocl_native_tanhf %35

However, current use of __spirv_ocl_native_tanh in SYCL header is restricted to __NVPTX__ path. So SPIR-V path won't see it. But people can still use it for SPIR-V since it is in public header.