Do no pollute the top level namespace

While playing with FPGA libraries we got some compiling conflicts with some `::ap_int` type.

It happens that while https://github.com/intel/llvm/blob/sycl/sycl/include/CL/__spirv/spirv_types.hpp declares things inside `::__spv` there are also some random declarations at the end of the file, like `RPipeTy`, `WPipeTy`, `ConstantPipeStorage`, `ap_int`.

If these declarations are useful, they should go into some detail or extension namespaces.

There are some other top-level declarations in `__spirv` but they are starting with `__` which is OK since it is a kind of private implementation world in C/C++ provided there is some collaborative damage control.