<rdar://problem/13064893>

Poor network connections aren't handled well; commands fail instead of retrying.
Add two new functions to the C API:

 LLVMCreateMemoryBufferWithMemoryRange - exposes MemoryBuffer::getMemBuffer
 LLVMCreateMemoryBufferWithMemoryRangeCopy - exposes MemoryBuffer::getMemBufferCopy

Patch by Moritz Maxeiner!