Warn when performing 'usual' conversions that require a sign change. This

implements test/Preprocessor/expr_usual_conversions.c, which produces this
output:

expr_usual_conversions.c:5:10: warning: left side of operator converted from negative value to unsigned: -42 to 18446744073709551574
#if (-42 + 0U) / -2
         ^
expr_usual_conversions.c:5:16: warning: right side of operator converted from negative value to unsigned: -2 to 18446744073709551614
#if (-42 + 0U) / -2
               ^

llvm-svn: 39406