rustllvm: Don't require null terminators in files

Apparently the default getFile implementation for a memory buffer in LLVM ends
up requiring a null terminator at the end of the file. This isn't true a good
bit of the time apparently on OSX. There have been a number of failed
nightly/snapshot builds recently with this strange assertion.

This modifies the calls to MemoryBuffer::getFile to explicitly not ask for a
null terminator.
I'm not quite sure, I don't think we have much style when it comes to these wrapper files. I think we still run the danger of compiling this file with pre-C++11 compilers though, so maybe not?