[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 aarch64 build error in FEnvSafeTest change (#89826)

[libc] Make fenv and math tests preserve fenv_t state (#89658)

This adds a new test fixture class FEnvSafeTest (usable as a base
class for other fixtures) that ensures each test doesn't perturb
th

[libc] Make fenv and math tests preserve fenv_t state (#89658)

This adds a new test fixture class FEnvSafeTest (usable as a base
class for other fixtures) that ensures each test doesn't perturb
the `fenv_t` state that the next test will start with. It also
provides types and methods tests can use to explicitly wrap code
under test either to check that it doesn't perturb the state or
to save and restore the state around particular test code.

All the fenv and math tests are updated to use this so that none
can affect another. Expectations that code under test and/or
tests themselves don't perturb state can be added later.

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