[UB] Another place where we were trying to put string data into

a BumpPtrAllocator. This at least now handles the case where there is no
concatentation without calling memcpy on a null pointer. It might be
interesting to handle the case where everything is empty without
round-tripping through the allocator, but it wasn't clear to me if the
pointer returned is significant in any way, so I've left it in
a conservatively more-correct state.

Again, found with UBSan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243948 91177308-0d34-0410-b5e6-96231b3b80d8