[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration (#98597)

This is a part of #97655.

Revert "[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration" (#98593)

Reverts llvm/llvm-project#98075

bots are broken

[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration (#98075)

This is a part of #97655.

[libc] Fix missing warp sync for the NVPTX assert

Summary:
The implementation of `assert` has an if statement so that only the
first thread in the warp prints the assertion. On modern NVPTX
architec

[libc] Fix missing warp sync for the NVPTX assert

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.

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[libc] Mass replace enclosing namespace (#67032)

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

[libc][NFC] Factor GPU exiting into a common function (#66093)

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

[libc][NFC] Factor GPU exiting into a common function (#66093)

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.

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[libc] Support 'assert.h' on the GPU

This patch adds the necessary support to provide `assert` functionality
through the GPU `libc` implementation. This implementation creates a
special-case GPU im

[libc] Support 'assert.h' on the GPU

This patch adds the necessary support to provide `assert` functionality
through the GPU `libc` implementation. This implementation creates a
special-case GPU implementation rather than relying on the common
version. This is because the GPU has special considerings for printing.
The assertion is printed out in chunks with `write_to_stderr`, however
when combined with the GPU execution model this causes 32+ threads to
all execute in-lock step. Meaning that we'll get a horribly fragmented
message. Furthermore, potentially thousands of threads could hit the
assertion at once and try to print even if we had it all in one
`printf`.

This is solved by having a one-time lock that each thread group / wave /
warp will attempt to claim. We only let one thread group pass through
while the others simply stop executing. Finally only the first thread in
that group will do the printing until we finally abort execution.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159296

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