GlobalStatus: Don't walk use-lists of ConstantData

Return early from llvm::isSafeToDestroyConstant() whenever the value
`isa<ConstantData>()`.  These constants are shared across the
LLVMContext.  We never really want to delete them here, and walking
their use-lists can be very expensive.

(This is motivated by an eventual goal of removing use-lists entirely
from ConstantData.)

llvm-svn: 282320