[libc++][NFC] Rename _LIBCPP_NO_EXCEPTIONS to _LIBCPP_HAS_NO_EXCEPTIONS

Other macros that disable parts of the library are named `_LIBCPP_HAS_NO_WHATEVER`.

Reviewed By: ldionne, Mordante, #libc

Spies: libcxx-commits, smeenai

Differential Revision: https://reviews.llvm.org/D143163