This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

It's meant to be included multiple times! Maybe use a NOLINT rule to
suppress
clang-tidy's llvm-header-guard lint warning.

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).

Summary:
The implementation of `assert` has an if statement so that only the
first thread in the warp prints the assertion. On modern NVPTX
architecture, this can be printed out of order with the abort call. This
would lead to only a portion of the message being printed and then
exiting the program. By adding a mandatory warp sync we force the full
string to be printed before we continue to the abort.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

Summary:
We currently call the GPU routine to terminate the current thread in
three separate locations .This should be wrapped into a helper function
to simplify the implementation.