[clang] [C++] Expose nullptr_t from stddef.h in C++ mode (PR #154599)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Thanks! I can try to investigate from my side but I'm on Windows and I don't typically build libc++ so it's possible I won't get very far. Due to timing, I may not get back to this before October, so assistance is very much appreciated.

https://github.com/llvm/llvm-project/pull/154599