[clang] Reapply "[clang] Fix some static initialization race-conditions" (PR #182020)

[Javier Lopez-Gomez via cfe-commits]

https://github.com/jalopezg-git approved this pull request.

LGTM ✅ -- thank you! (I see the issue was that `Table` was default-initialized, and with `StmtClassNameTable` being a POD type, (1) unassigned array elements stay uninitialized; and (2) the initial value for `Counter` is undefined for all elements in `Table`)

https://github.com/llvm/llvm-project/pull/182020