libcxx-compat: revert llvmorg-19-init-17473-g69fecaa1a455:

[libc++] Clean up some now dead code with the upgrade to GCC 14 (#97746)

PR: 280562
MFC after: 1 month

Merge llvm-project main llvmorg-19-init-18630-gf2ccf80136a0

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-19-init-18630-gf2ccf80136a0, t

Merge llvm-project main llvmorg-19-init-18630-gf2ccf80136a0

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-19-init-18630-gf2ccf80136a0, the
last commit before the upstream release/19.x branch was created.

PR: 280562
MFC after: 1 month

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