[libcxx] [test] D27020: Fix MSVC warning C4245 "conversion from 'X' to 'Y',...

[libcxx] [test] D27020: Fix MSVC warning C4245 "conversion from 'X' to 'Y', signed/unsigned mismatch", part 7/12.

When initializing unsigned integers to their maximum values, change "const T M(~0);" to "const T M(static_cast<T>(-1));".

~0 and -1 are equivalent, but I consider the -1 form to be significantly clearer (and more consistent with other tests).

llvm-svn: 287827