Disable MSVC 2015's warning about zero extending after ~ and others

Very often in LLVM we have APIs that take a bitwidth and a uint64_t that
we pass immediates such as ~0U to. Consider APInt, Constant, and
MachineInstrBuilder::addImm. Fixing all uses of these APIs to manually
extend their arguments to uint64_t doesn't seem worth it.

llvm-svn: 260416