Make -Wabsolute-value C++-aware.

Warn on std::abs() with unsigned argument.
Suggest std::abs as replacement for the C absolute value functions.
Suggest C++ headers if the specific std::abs overload is not found.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206340 91177308-0d34-0410-b5e6-96231b3b80d8