Have the UTF conversion wrappers append a null terminator.

Have the UTF conversion wrappers append a null terminator.

This is especially useful for the UTF8 -> UTF16 direction, since
there is no equivalent of llvm::SmallString<> for wide characters.
This means that anyone who wants a null terminated string is forced
to manually push and pop their own null terminator.

Reviewed by: Reid Kleckner.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227143 91177308-0d34-0410-b5e6-96231b3b80d8