Diagnostics are now emitted via the SourceMgr and we use MemoryBuffer
for buffer management. Switched the code to make use of the trailing
'0' that MemoryBuffer guarantees where it makes sense.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147063 91177308-0d34-0410-b5e6-96231b3b80d8
