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.
Switch from {}-initialization syntax to () function call syntax, which
will allow the buf.second to be either signed or unsigned.

Bug: b/175635923
Test: treehugger
Change-Id: I6897c621ae48683a2873ce156a9064edacdff909
1 file changed