[patch] Pass a MemoryBufferRef when we can avoid taking ownership

[Rafael Espíndola]

Hi David,

The attached patch simplifies a few interfaces that don't need to take
ownership of a buffer.

For example, both parseAssembly and parseBitcodeFile will parse the
entire buffer before returning. There is no need to take ownership.

Using a MemoryBufferRef makes it obvious in the type signature that
there is no ownership transfer.

Cheers,
Rafael
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