Accommodate a change in the type of std::span's size

The WIP version of std::span in external/libcxx uses a ptrdiff_t size,
but the final standardized version of std::span uses size_t instead.
Use std::span() constructor calls rather than {}-syntax, which will
convert the signed length to unsigned and works with either the old or
the new libc++.

Test: treehugger
Change-Id: I5b5a16d0949e77a74269b9f6cf24382dd69a5973