[WinEH] Emit .safeseh directives for all 32-bit exception handlers

[WinEH] Emit .safeseh directives for all 32-bit exception handlers

Use a "safeseh" string attribute to do this. You would think we chould
just accumulate the set of personalities like we do on dwarf, but this
fails to account for the LSDA-loading thunks we use for
__CxxFrameHandler3. Each of those needs to make it into .sxdata as well.
The string attribute seemed like the most straightforward approach.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239448 91177308-0d34-0410-b5e6-96231b3b80d8