Revert "Revert 241171, 241187, 241199 (32-bit SEH)."

This reverts commit r241244, but restricts SEH support to Win64.

This way, Chromium builds will still fall back on TUs with SEH, and
Clang developers can work on this incrementally upstream while patching
this small predicate locally. It'll also make it easier to review small
fixes.

llvm-svn: 241533