This reverts commit 83a798d4b0e17ac41d5430f1290d3661343eee1e.
As discussed in D120714 with @thakis, the patch added unneeded complexity
without noticeable benefits.
Place PersistentId declaration under #if LLVM_ENABLE_ABI_BREAKING_CHECKS to
reduce memory usage when it is not needed.
Differential Revision: https://reviews.llvm.org/D120714