[libc++] P2770R0: Stashing stashing iterators for proper flattening (#66033)

- Partially implements P2770R0 (http://wg21.link/p2770)
- Fixes https://wg21.link/LWG3698, https://wg21.link/LWG3700, an

[libc++] P2770R0: Stashing stashing iterators for proper flattening (#66033)

- Partially implements P2770R0 (http://wg21.link/p2770)
- Fixes https://wg21.link/LWG3698, https://wg21.link/LWG3700, and https://wg21.link/LWG3791
- join_with_view hasn't been done yet since this type isn't implemented yet
- Rename tuple test directory to match the standard (which changed in P2770R0)
- Rename join_view test directory to match the standard

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[libc++] Fix _CopySegment helper in ranges::copy(join_view, out) when called in a static assertion context (#69593)

Resolves Issue #69083

The `_CopySegment` helper for `ranges::copy(join_view, ou

[libc++] Fix _CopySegment helper in ranges::copy(join_view, out) when called in a static assertion context (#69593)

Resolves Issue #69083

The `_CopySegment` helper for `ranges::copy(join_view, out)` is not
`constexpr` causing rejection in `libc++` in a static assertion context
as in the issue snippet.

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[libc++][NFC] Remove spurious check for is-constant-evaluated

This NFC change was reverted as part of 880fa7faa97, but the change is
really good regardless of the associated Clang patch.

Revert "[clang][SemaCXX] Diagnose tautological uses of consteval if and is_constant_evaluated"

This reverts commit 491b2810fb7fe5f080fa9c4f5945ed0a6909dc92.

This change broke valid code and generat

Revert "[clang][SemaCXX] Diagnose tautological uses of consteval if and is_constant_evaluated"

This reverts commit 491b2810fb7fe5f080fa9c4f5945ed0a6909dc92.

This change broke valid code and generated incorrect diagnostics, see
https://reviews.llvm.org/D155064

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[clang][SemaCXX] Diagnose tautological uses of consteval if and is_constant_evaluated

This patch makes clang diagnose extensive cases of consteval if and is_constant_evaluated usage that are tautolo

[clang][SemaCXX] Diagnose tautological uses of consteval if and is_constant_evaluated

This patch makes clang diagnose extensive cases of consteval if and is_constant_evaluated usage that are tautologically true or false.
This introduces a new IsRuntimeEvaluated boolean flag to Sema::ExpressionEvaluationContextRecord that means the immediate appearance of if consteval or is_constant_evaluated are tautologically false(e.g. inside if !consteval {} block or non-constexpr-qualified function definition body)
This patch also pushes new expression evaluation context when parsing the condition of if constexpr and initializer of constexpr variables so that Sema can be aware that the use of consteval if and is_consteval are tautologically true in if constexpr condition and constexpr variable initializers.
BEFORE this patch, the warning for is_constant_evaluated was emitted from constant evaluator. This patch moves the warning logic to Sema in order to diagnose tautological use of is_constant_evaluated in the same way as consteval if.

This patch separates initializer evaluation context from InitializerScopeRAII.
This fixes a bug that was happening when user takes address of function address in initializers of non-local variables.

Fixes https://github.com/llvm/llvm-project/issues/43760
Fixes https://github.com/llvm/llvm-project/issues/51567

Reviewed By: cor3ntin, ldionne
Differential Revision: https://reviews.llvm.org/D155064

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[libc++] Fix modules issues on OS X

First, fix a collision with the Point type from MacTypes.h, which was
reported on Slack, 2022-07-31: https://cpplang.slack.com/archives/C2X659D1B/p165928469127588

[libc++] Fix modules issues on OS X

First, fix a collision with the Point type from MacTypes.h, which was
reported on Slack, 2022-07-31: https://cpplang.slack.com/archives/C2X659D1B/p1659284691275889

Second, rename the meta:: namespace to types::. OSX's "/usr/include/ncurses.h"
defines a `meta` function, and is (for some reason) included in
"<SDK>/usr/include/module.modulemap", so that identifier is off-limits
for us to use in anything that compiles with -fmodules:

libcxx/test/support/type_algorithms.h:16:11: error: redefinition of 'meta' as different kind of symbol
namespace meta {
^
<SDK>/usr/include/ncurses.h:603:28: note: previous definition is here
extern NCURSES_EXPORT(int) meta (WINDOW *,bool); /* implemented */
^

Finally, add a CI configuration for modules on OS X to make sure it
does not regress.

Differential Revision: https://reviews.llvm.org/D144915

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[libc++][ranges] Temporarily mark `ranges::join_view` as experimental.

This is in anticipation of https://isocpp.org/files/papers/D2770R0.html,
which is ABI-breaking for `join_view`.

Differential R

[libc++][ranges] Temporarily mark `ranges::join_view` as experimental.

This is in anticipation of https://isocpp.org/files/papers/D2770R0.html,
which is ABI-breaking for `join_view`.

Differential Revision: https://reviews.llvm.org/D142520

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[libc++] Enable segmented iterator optimizations for join_view::iterator

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D138413

[libc++] Refactor deque::iterator algorithm optimizations

This has multiple benefits:
- The optimizations are also performed for the `ranges::` versions of the algorithms
- Code duplication is reduc

[libc++] Refactor deque::iterator algorithm optimizations

This has multiple benefits:
- The optimizations are also performed for the `ranges::` versions of the algorithms
- Code duplication is reduced
- it is simpler to add this optimization for other segmented iterators,
like `ranges::join_view::iterator`
- Algorithm code is removed from `<deque>`

Reviewed By: ldionne, huixie90, #libc

Spies: mstorsjo, sstefan1, EricWF, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D132505

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