incfs: tolerate either ptrdiff_t or size_t for std::span

The out-of-date std::span in external/libcxx currently uses ptrdiff_t
as the type of size() and the second ctor argument. The C++20 std::span
actually uses size_t for these values instead. Try to accommodate both
versions of std::span until libc++ is upgraded.

Bug: http://b/175635923
Test: treehugger
Change-Id: Idc43f880b6eb21dad79e79fa456847d5865a1ed2