[clang] [Serialization] Load Specializations Lazily (PR #76774)

[David Blaikie via cfe-commits]

dwblaikie wrote:

@ilya-biryukov any chance you/your folks could test this change for performance implications in google? It's especially helpful to CERN, but the last iteration of this direction had some regressions that stalled out progress on that version a few years ago, so it'd be good to help poke this along while making sure it doesn't cause release hiccups/etc for google.

https://github.com/llvm/llvm-project/pull/76774