[coff] correctly emit safeseh entries for handlers defined in dlls

Summary:
We previously assumed that all SafeSEH handlers are
DefinedRegular symbols. This is not the case for handlers defined in
DLLs. As a result, we were failing to emit entries in the SafeSEH
table for those handlers. This change fixes that.

Fixes PR35324.

Reviewers: rnk, ruiu

Reviewed By: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40102

llvm-svn: 318364