[ORC][MachO] Fix MachOPlatform after 9189a266.

Commit 9189a266 caused llvm-jitlink to create bare JITDylibs to wrap real
dylibs loaded via -preload. This exposed a bug in MachOPlatform where we
assumed that all JITDylibs had been registered with the platform through
MachOPlatform::setupJITDylib (bare JITDylibs are _not_ run through this
function), and errored out where this was not the case.
This bug in MachOPlatform was causing test failures in compilert-rt:

Failed Tests (2):
  ORC-x86_64-darwin :: TestCases/Darwin/x86-64/trivial-objc-methods.S
  ORC-x86_64-darwin :: TestCases/Darwin/x86-64/trivial-swift-types-section.S

This commit fixes the issue by skipping JITDylibs that haven't been registered
with the platform via MachOPlatform::setupJITDylib.