[libc++] Fix constexpr dynamic allocation on GCC 10

We're technically not allowed by the Standard to call ::operator new in
constexpr functions like __libcpp_allocate. Clang doesn't seem to complain
about it, but GCC does.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: d092c912885cc152bef27019525b8fd0761aaaa2
4 files changed