[libcxx] [test] D27019: Fix MSVC warning C4245 "conversion from 'X' to 'Y',...

[libcxx] [test] D27019: Fix MSVC warning C4245 "conversion from 'X' to 'Y', signed/unsigned mismatch", part 6/12.

Add static_cast when initializing unsigned integers with negative numbers (in order to obtain big values).

llvm-svn: 287826