[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).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@287826 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters.arithmetic/unsigned_long_long.pass.cpp

@@ -76,7 +76,7 @@ int main()
         testbuf<char> sb;
         std::ostream os(&sb);
         hex(os);
-        unsigned long long n = -10;
+        unsigned long long n = static_cast<unsigned long long>(-10);
         os << n;
         assert(sb.str() == "fffffffffffffff6");
     }