Constrain fill_n -> memset operations to include implicit convertibility to unsigned char.  This fixes http://llvm.org/bugs/show_bug.cgi?id=16764.  Also a drive-by fix on a chrono test suite bug.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@187552 91177308-0d34-0410-b5e6-96231b3b80d8