Warn on unsigned zero in call to std::max

New default warning that triggers when an unsigned zero is used in a call to
std::max.  For unsigned values, zero is the minimum value, so any call to
std::max is always equal to the other value.  A common pattern was to take
the max of zero and the difference of two unsigned values, not taking into
account that unsigned values wrap around below zero.  This warning also emits
a note with a fixit hint to remove the zero and call to std::max.

llvm-svn: 288732