[Support] Add MemoryBuffer::dontNeedIfMmap

On *NIX systems, this API calls madvise(MADV_DONTNEED) on read-only file mappings.
It should not be used on a writable buffer.
The API is used to implement ld.lld LTO memory saving trick (D116367).

Note: on read-only file mappings, Linux's MADV_DONTNEED semantics match POSIX
POSIX_MADV_DONTNEED and BSD systems' MADV_DONTNEED.

On Windows, VirtualAllocEx MEM_COMMIT/MEM_RESET have similar semantics
but are unfortunately not drop-in replacements. dontNeedIfMmap is currently a no-op.

Reviewed By: aganea

Differential Revision: https://reviews.llvm.org/D116366