llvm::sys::SwapByteOrder() renamed to llvm::sys::getSwappedBytes()

Further to this, llvm::sys::swapByteOrder() will be added, acting in-place



git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210974 91177308-0d34-0410-b5e6-96231b3b80d8