MS ABI: Simplify endian swapping code

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205250 91177308-0d34-0410-b5e6-96231b3b80d8
MS ABI: Add a test for Unicode literals

This adds coverage for Unicode code points which are encoded with
non-zero values in the upper half of the wchar_t.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205251 91177308-0d34-0410-b5e6-96231b3b80d8