[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++] 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++] Implement ranges::contains (#65148)

Differential Revision: https://reviews.llvm.org/D159232
```
Running ./ranges_contains.libcxx.out
Run on (10 X 24.121 MHz CPU s)
CPU Caches:
L1 Dat

[libc++] Implement ranges::contains (#65148)

Differential Revision: https://reviews.llvm.org/D159232
```
Running ./ranges_contains.libcxx.out
Run on (10 X 24.121 MHz CPU s)
CPU Caches:
L1 Data 64 KiB (x10)
L1 Instruction 128 KiB (x10)
L2 Unified 4096 KiB (x5)
Load Average: 3.37, 6.77, 5.27
--------------------------------------------------------------------
Benchmark Time CPU Iterations
--------------------------------------------------------------------
bm_contains_char/16 1.88 ns 1.87 ns 371607095
bm_contains_char/256 7.48 ns 7.47 ns 93292285
bm_contains_char/4096 99.7 ns 99.6 ns 7013185
bm_contains_char/65536 1296 ns 1294 ns 540436
bm_contains_char/1048576 23887 ns 23860 ns 29302
bm_contains_char/16777216 389420 ns 389095 ns 1796
bm_contains_int/16 7.14 ns 7.14 ns 97776288
bm_contains_int/256 90.4 ns 90.3 ns 7558089
bm_contains_int/4096 1294 ns 1290 ns 543052
bm_contains_int/65536 20482 ns 20443 ns 34334
bm_contains_int/1048576 328817 ns 327965 ns 2147
bm_contains_int/16777216 5246279 ns 5239361 ns 133
bm_contains_bool/16 2.19 ns 2.19 ns 322565780
bm_contains_bool/256 3.42 ns 3.41 ns 205025467
bm_contains_bool/4096 22.1 ns 22.1 ns 31780479
bm_contains_bool/65536 333 ns 332 ns 2106606
bm_contains_bool/1048576 5126 ns 5119 ns 135901
bm_contains_bool/16777216 81656 ns 81574 ns 8569
```

---------

Co-authored-by: Nathan Gauër <brioche@google.com>

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