This commits moves all OpenCL builtins under a top-level 'opencl'
directory, akin to how the CLC builtins are organized. This new
structure aims to better convey the separation of the two layers and
that 'CLC' is not a subset of OpenCL or a libclc target.
In doing so this commit moves the location of the 'lib' directory to
match CLC: libclc/generic/lib/ becomes libclc/opencl/lib/generic/. This
allows us to remove some special casing in CMake and ensure a common
directory structure.
It also tries to better communicate that the OpenCL headers are
libclc-specific OpenCL headers and should not be confused with or used
as standard OpenCL headers. It does so by ensuring includes are of the
form <clc/opencl/*>. It might be that we don't specifically need the
libclc OpenCL headers and we simply could use clang's built-in
declarations, but we can revisit that later.
Aside from the code move, there is some code formatting and updating a
couple of OpenCL builtin includes to use the readily available gentype
helpers. This allows us to remove some '.inc' files.
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