This patch adds a hack to clang's emitPointerArithmetic() function to get it to emit an inttoptr instruction rather than a GEP with a null base pointer when the 'p = nullptr + n' idiom is used to convert a pointer-sized integer to a pointer. This idiom is used by some versions of glibc and gcc.
This was previously discussed here: http://lists.llvm.org/pipermail/llvm-dev/2017-July/115053.html
"extension" isn't really right here; this shouldn't be an error in -pedantic-errors mode. Probably best to just stick this into the NullPointerArithmetic, like the other new warning.