[ORC][MachO] Simplify use of LC_BUILD_VERSION in JITDylib headers.

API clients can now set a MachO::HeaderOptions::BuildVersionOpts field to have
MachOPlatform add an LC_BUILD_VERSION load command to the Mach header for each
JITDylib.

No testcase yet. In the future we'll try to add a MachO parser to the ORC
runtime and extra test options to llvm-jitlink for this.

This commit also incidentally fixes a bug in the MachOBuilder class that lead to
a delegation cycle.