[llvm] [MCParser] .incbin: Don't retain the buffer, don't require NUL termination (PR #196696)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I'll need to restore the previous approach that adds the `RequiresNullTerminator` parameter to `SourceMgr::OpenIncludeFile`, since `llvm/lib/TableGen/TGLexer.cpp`, another user of the API, has a non-trivial NUL terminator requirement.

https://github.com/llvm/llvm-project/pull/196696