[PECOFF] Implement /SAFESEH option.

LLD now prints an error message if /SAFESEH option is specified and one or
more input files are not compatible with SEH.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@201900 91177308-0d34-0410-b5e6-96231b3b80d8