[libcxx] [test] Silence MSVC warning C4146.

This test code triggers the MSVC warning:

"unary minus operator applied to unsigned type, result still unsigned"

Although it would be possible to change the test code to avoid
this warning, I have chosen to simply silence it.

Fixes D45594.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@329976 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed