[ORC] loadRelocatableObject: universal binary support, clearer errors (#104406)

ORC supports loading relocatable object files into a JIT'd process. The
raw "add object file" API (ObjectLayer::add) accepts plain relocatable
object files as llvm::MemoryBuffers only and does not check that the
object file's format or architecture are compatible with the process
that it will be linked in to. This API is flexible, but places the
burden of error checking and universal binary support on clients.

This commit introduces a new utility, loadRelocatableObject, that takes
a path to load and a target triple and then:
1. If the path does not exist, returns a FileError containing the
invalid path.
2. If the path points to a MachO universal binary, identifies and
returns MemoryBuffer covering the slice that matches the given triple
(checking that the slice really does contains a valid MachO relocatable
object with a compatible arch).
3. If the path points to a regular relocatable object file, verifies
that the format and architecture are compatible with the triple.

Clients can use loadRelocatableObject in the common case of loading
object files from disk to simplify their code.

Note: Error checking for ELF and COFF is left as a FIXME.

rdar://133653290

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