Fix possible loss of data warnings on amd64

In T_size_size.pass, there is an explicit template argument to std::min to ask
for unsigned, to avoid type deduction errors. However, C1XX' warnings still
hate this use, because a 64 bit value (a size_t) is being passed to a function
accepting an unsigned (a 32 bit value).

Instead, change the tests to pass around std::size_t instances, and explicitly
narrow when constructing the string type under test. This also allows
removal of explicit template arguments to std::min.

llvm-svn: 302473