[rtsan][test] Prevent test check from being optimized out in LTO builds (#122524)

In LTO builds, some test checks can be optimized away, since the
compiler can
see through the memory accesses afte

[rtsan][test] Prevent test check from being optimized out in LTO builds (#122524)

In LTO builds, some test checks can be optimized away, since the
compiler can
see through the memory accesses after inlining across TUs. This causes
the existing death tests to fail, since the functions are completely
optimized out and things like copying a lambda will no longer occur and
trigger the sanitizer.

To prevent that, we can use an empty inline assembly block to tell the
compiler that memory is modified, and prevent it from doing that.

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[rtsan] Add sched_yield interceptor (#117084)

This calls the system calls switch_pri and sys_ulock_wait. It also is
one of the more straightforwardly rt-unsafe, in that it gives up this
thread's t

[rtsan] Add sched_yield interceptor (#117084)

This calls the system calls switch_pri and sys_ulock_wait. It also is
one of the more straightforwardly rt-unsafe, in that it gives up this
thread's timeslice.

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[rtsan][llvm][NFC] Rename sanitize_realtime_unsafe attr to sanitize_realtime_blocking (#113155)

# What

This PR renames the newly-introduced llvm attribute
`sanitize_realtime_unsafe` to `sanitize

[rtsan][llvm][NFC] Rename sanitize_realtime_unsafe attr to sanitize_realtime_blocking (#113155)

# What

This PR renames the newly-introduced llvm attribute
`sanitize_realtime_unsafe` to `sanitize_realtime_blocking`. Likewise,
sibling variables such as `SanitizeRealtimeUnsafe` are renamed to
`SanitizeRealtimeBlocking` respectively. There are no other functional
changes.


# Why?

- There are a number of problems that can cause a function to be
real-time "unsafe",
- we wish to communicate what problems rtsan detects and *why* they're
unsafe, and
- a generic "unsafe" attribute is, in our opinion, too broad a net -
which may lead to future implementations that need extra contextual
information passed through them in order to communicate meaningful
reasons to users.
- We want to avoid this situation and make the runtime library boundary
API/ABI as simple as possible, and
- we believe that restricting the scope of attributes to names like
`sanitize_realtime_blocking` is an effective means of doing so.

We also feel that the symmetry between `[[clang::blocking]]` and
`sanitize_realtime_blocking` is easier to follow as a developer.

# Concerns

- I'm aware that the LLVM attribute `sanitize_realtime_unsafe` has been
part of the tree for a few weeks now (introduced here:
https://github.com/llvm/llvm-project/pull/106754). Given that it hasn't
been released in version 20 yet, am I correct in considering this to not
be a breaking change?

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[rtsan][NFC] Move away from system include style for local headers (#109977)

[rtsan][NFC] Standardize lambda function case, fix autos (#109541)

[compiler-rt][rtsan] NFC: Rename rtsan_on->rtsan_enable rtsan_off->rtsan_disable (#106727)

This better matches lsan_enable and disable, which we are trying to
emulate.

[compiler-rt][rtsan] Introduce first end to end RTsan lit tests, enable instrumented unit tests (#105732)

[compiler-rt] Realtime Sanitizer: Introduce Realtime Sanitizer (RTSan) backend (#92460)

Introducing the main runtime of realtime sanitizer. For more
information, please see the [discourse
thread](

[compiler-rt] Realtime Sanitizer: Introduce Realtime Sanitizer (RTSan) backend (#92460)

Introducing the main runtime of realtime sanitizer. For more
information, please see the [discourse
thread](https://discourse.llvm.org/t/rfc-nolock-and-noalloc-attributes/76837).
We have also put together a [reviewer support
document](https://github.com/realtime-sanitizer/radsan/blob/doc/review-support/doc/review.md)
to show what our intention is.

This review introduces the sanitizer backend. This includes:
* CMake build files (largely adapted from asan).
* Main RTSan architecture (the external API, thread local context,
stack).
* Interceptors.
* Many unit tests.

Please see the [reviewer support
document](https://github.com/realtime-sanitizer/radsan/blob/doc/review-support/doc/review.md)
for what our next steps are. We are moving in lockstep with this PR
#84983 for the codegen coming up next.

Note to reviewers: If you see support documentation mention "RADSan",
this was the "old acronym" for the realtime sanitizer, they refer to the
same thing. If you see it let us know and we can correct it (especially
in the llvm codebase)

---------

Co-authored-by: David Trevelyan <david.trevelyan@gmail.com>

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