[libc++] P3372R3: constexpr `deque`

[frederick-vs-ja]

Make deque constexpr as part of P3372R3.

Feature-test macro:

#define __cpp_lib_constexpr_deque 202502L // also in <deque>

[winner245]

For tracking purpose: There are tests involving deque that currently cannot be marked as TEST_CONSTEXPR_CXX23 due to lack of constexpr support for deque. As a follow-up to making std::deque constexpr, these tests would need to be updated to be TEST_CONSTEXPR_CXX23TEST_CONSTEXPR_CXX26, and the condition becomes TEST_STD_AT_LEAST_26_OR_RUNTIME_EVALUATED. The affected tests include:

• PR: [libc++] Optimize ranges::{for_each, for_each_n} for segmented iterators #132896
  libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/ranges.for_each.pass.cpp
libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/for_each_n.pass.cpp
libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/ranges.for_each_n.pass.cpp
libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/for_each.pass.cpp

• PR: [libc++] Optimize {std,ranges}::{fill,fill_n} for segmented iterators #132665
  libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp
libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp
libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp
libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp

[frederick-vs-ja]

For tracking purpose: There are tests involving deque that currently cannot be marked as TEST_CONSTEXPR_CXX23 due to lack of constexpr support for deque. As a follow-up to making std::deque constexpr, these tests would need to be updated to be TEST_CONSTEXPR_CXX23.

IIUC it is TEST_CONSTEXPR_CXX26 & TEST_STD_AT_LEAST_26_OR_RUNTIME_EVALUATED that should be used.