[MCParser] .incbin: Don't retain the buffer, don't require NUL termination (#196696)
processIncbinFile uses SourceMgr::AddIncludeFile, which
* sets `RequiresNullTerminator=true` and disable `mmap` when the file
size is a multiple of the page size,
* and unnecessarily retains the throwaway buffer in `Buffers`.
Switch to OpenIncludeFile so the buffer is freed when processIncbinFile
returns, and pass RequiresNullTerminator=false. The buffer is consumed
only by emitBytes; the lexer never scans it, so it does not need a
trailing '\0' (different from #154972). Without that requirement,
MemoryBuffer mmaps the file and RSS tracks only the touched pages.
Stress test (1000 .incbin "blob.bin", 0, 16 against a 1 MiB blob):
```
Maximum RSS
Before 1042944 KiB
After 15360 KiB
```
Fix #62339
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