Change PlatformDarwinKernel::GetSharedModule to be a little more

explicit in how it adds the kernel binary, to guard against the
case where a kernel corefile might incorrectly include the kernel's
UUID in it (so calling ::GetSharedModule may end up returning the
global module cache's copy of the core file instead of adding the
kerenl binary).

<rdar://problem/26988816> 

llvm-svn: 273954