[ADT] APInt.h - remove <cmath> include. NFC.

We only need this for std::abs, but since we're also testing the sign of the same value, then its not really necessary.

As detailed in https://commondatastorage.googleapis.com/chromium-browser-clang/llvm-include-analysis.html - APInt.h is the generic header with the highest expanded size, due to the dependency on <cmath>