Summary:
Precommit for later use.

Summary:
These helpers are very useful but currently absent. They allow the user
to get a bitmask representing the matches within the warp. I have made
an executive decision to drop the `predicate` return from `match_all`
because it's easily testable with `match_all() == __activemask()`.

Summary:
The CUDA impelementation has long supported the `width` argument on its
shuffle instrucitons, which makes it more difficult to replace those
uses with this helper. This patch just correctly implements that for
AMDGPU and NVPTX so it's equivalent to `__shfl_sync` in CUDA. This will
ease porting.

Fortunately these get optimized out correctly when passing in known
widths.

Summary:
This patch switches the GPU utility helpers to wrapping around the
gpuintrin.h ones with a C++ flavor.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

src/__support/GPU/utils.h doesn't compile on a 32-bit platforms because
__builtin_ffsl uses long which is a 32-bit number. Use __builtin_ffsll
which uses long long which is guaranteed to be at least 64-bits.

Summary:
The GPU uses a SIMT execution model. That means that each value actually
belongs to a group of 32 or 64 other lanes executing next to it. These
platforms offer some intrinsic fuctions to actually take elements from
neighboring lanes. With these we can do parallel scans or reductions.
These functions do not have an immediate user, but will be used in the
allocator interface that is in-progress and are generally good to have.
This patch is a precommit for these new utilitly functions.

Towards the goal of getting `ninja libc-lint` back to green, fix the numerous
instances of:

    warning: header guard does not follow preferred style [llvm-header-guard]

This is because many of our header guards start with `__LLVM` rather than
`LLVM`.

To filter just these warnings:

    $ ninja -k2000 libc-lint 2>&1 | grep llvm-header-guard

To automatically apply fixits:

    $ find libc/src libc/include libc/test -name \*.h | \
        xargs -n1 -I {} clang-tidy {} -p build/compile_commands.json \
        -checks='-*,llvm-header-guard' --fix --quiet

Some manual cleanup is still necessary as headers that were missing header
guards outright will have them inserted before the license block (we prefer
them after).