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.
auto merge of #15779 : alexcrichton/rust/no-nul-terminator, r=pcwalton

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.