This doesn't need to pretend to support multiple versions of llvm
and these are old anyway.

Fix `libclc/utils/CMakeLists.txt` to expose `prepare_builtins_*`
variables in parent scope. This was a regression introduced in #148815
where the code was moved into subdirectory, and the variables would no
longer be accessible to calls in top-level CMakeLists, resulting in
attempting to build targets with empty command:

```
[1566/1676] cd /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build && -o /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/obj.libclc.dir/clspv--/builtins.opt.clspv--.bc
FAILED: clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc
cd /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build && -o /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/obj.libclc.dir/clspv--/builtins.opt.clspv--.bc
/bin/sh: line 1: -o: command not found
```

This simply makes things better self-contained.

This commit finishes the work started in #146840 and #147276. It makes
each OpenCL header self-contained and each implementation file include
only the headers it needs. It removes the need for a catch-all include
file of all OpenCL builtin declarations.

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.

This commit bulk updates all '.h', '.cl', '.inc', and '.cpp' files to
add any missing license headers.

The remaining files are generally CMake, SOURCES, scripts, markdown,
etc.

There are still some '.ll' files which may benefit from a license
header. I can't find an example of an LLVM IR file with a license header
in the rest of LLVM, but unlike most other (sub)projects, libclc has
examples of LLVM IR as source files, compiled and built into the
library.

Stop using the compatibility spellings of `OF_{None,Text,Append}`
left behind by 1f67a3cba9b09636c56e2109d8a35ae96dc15782. A follow-up
will remove them.

Differential Revision: https://reviews.llvm.org/D101650

This reverts commit 58c814614d2ac69bcf79b09543505fac80ada4e6.
Fixes build breakage using LLVM<7.

llvm-svn: 367893

F_{None,Text,Append} are kept for compatibility since r334221.

llvm-svn: 367800