[libc++] Remove dedicated namespaces for ranges functions (#76543)

We originally put implementation-detail function objects into individual
namespaces for `std::ranges` without a good reason for do

[libc++] Remove dedicated namespaces for ranges functions (#76543)

We originally put implementation-detail function objects into individual
namespaces for `std::ranges` without a good reason for doing so. This
practice was continued, presumably because there was prior art. Since
there's no reason to keep these namespaces, this commit removes them,
which will slightly impact binary size.

This commit does not apply to CPOs, some of which need additional work.

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[libc++] Remove _LIBCPP_DISABLE_NODISCARD_EXTENSIONS and refactor the tests (#87094)

This also adds a few tests that were missing.

[libc++][ranges] use `static operator()` for C++23 ranges (#86052)

## Abstract

This pull request converts the `operator()` of all CPOs and niebloids
related to C++23 ranges to `static`.

## Mo

[libc++][ranges] use `static operator()` for C++23 ranges (#86052)

## Abstract

This pull request converts the `operator()` of all CPOs and niebloids
related to C++23 ranges to `static`.

## Motivation

In `libc++`, CPOs and niebloids are implemented as function objects.
Currently, the `operator()` for such a function object is a
`const`-qualified member function. This means that even if the function
object is has no data members, an extra register is used to pass in the
`this` pointer when calling `operator()`, unless the compiler can inline
the function call. Declaraing `operator()` as `static` would optimize
away the unnecessary `this` pointer passing for stateless function
objects, since there is no object instance state that needs to be
accessed.

## Reference

- [P1169R4: static `operator()`](https://wg21.link/P1169R4)

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[libc++] Fix missing and incorrect push/pop macros (#79204)

We recently noticed that the unwrap_iter.h file was pushing macros, but
it was pushing them again instead of popping them at the end of t

[libc++] Fix missing and incorrect push/pop macros (#79204)

We recently noticed that the unwrap_iter.h file was pushing macros, but
it was pushing them again instead of popping them at the end of the
file. This led to libc++ basically swallowing any custom definition of
these macros in user code:

#define min HELLO
#include <algorithm>
// min is not HELLO anymore, it's not defined

While investigating this issue, I noticed that our push/pop pragmas were
actually entirely wrong too. Indeed, instead of pushing macros like
`move`, we'd push `move(int, int)` in the pragma, which is not a valid
macro name. As a result, we would not actually push macros like `move`
-- instead we'd simply undefine them. This led to the following code not
working:

#define move HELLO
#include <algorithm>
// move is not HELLO anymore

Fixing the pragma push/pop incantations led to a cascade of issues
because we use identifiers like `move` in a large number of places, and
all of these headers would now need to do the push/pop dance.

This patch fixes all these issues. First, it adds a check that we don't
swallow important names like min, max, move or refresh as explained
above. This is done by augmenting the existing
system_reserved_names.gen.py test to also check that the macros are what
we expect after including each header.

Second, it fixes the push/pop pragmas to work properly and adds missing
pragmas to all the files I could detect a failure in via the newly added
test.

rdar://121365472

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[libc++][NFC] Use angle brackets to include ranges_mismatch.h

[libc++] Implement ranges::starts_with