"private" symbols which the assember shouldn't strip, but which the linker may
remove after evaluation. This is mostly useful for Objective-C metadata.
This is plumbing, so we don't have a use of it yet. More to come, etc.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@76385 91177308-0d34-0410-b5e6-96231b3b80d8