[RuntimeDyld] Plumb Error/Expected through the internals of RuntimeDyld.

Also replaces a number of calls to report_fatal_error with Error returns.

The plumbing will make it easier to return errors originating in libObject.

Replacing report_fatal_errors with Error returns will give JIT clients the
opportunity to recover gracefully when the JIT is unable to produce/relocate
code, as well as providing meaningful error messages that can be used to file
bug reports.

llvm-svn: 267776