Summary:
Add error messages to a couple of static_asserts in span to match the
style used in the rest of the file. Also fix an extra paren typo in a
assert error message.
Committed on behalf of Jason Lovett.
Reviewers: ldionne
Subscribers: libcxx-commits
Differential Revision: https://reviews.llvm.org/D52841
llvm-svn: 343725