Revert "[clang] Avoid memcopy for small structure with padding under...

Revert "[clang] Avoid memcopy for small structure with padding under -ftrivial-auto-var-init (#71677)"

This reverts commit 0d2860b7.

This change appears to have broken several clang tests on following buildbots:
https://lab.llvm.org/buildbot/#/builders/245
https://lab.llvm.org/buildbot/#/builders/188
https://lab.llvm.org/buildbot/#/builders/186
https://lab.llvm.org/buildbot/#/builders/183