[libc++][span] SFINAE span default constructor on Extent == 0

The default constructor of a static span requires _Extent == 0 so
SFINAE it out rather than using a static_assert

Differential Revision: https://reviews.llvm.org/D71994

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 79941086fba3df115e5e7b54a5f826096c933102
3 files changed