Adding llvm::sys::swapByteOrder() for the common use-case of byte-swapping a value in place

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210976 91177308-0d34-0410-b5e6-96231b3b80d8
Using llvm::sys::swapByteOrder() for the common case of byte-swapping a value in place

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210978 91177308-0d34-0410-b5e6-96231b3b80d8
Replacing the private implementations of SwapValue with calls to sys::swapByteOrder()

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210980 91177308-0d34-0410-b5e6-96231b3b80d8