[WinCOFF] Add support for the .safeseh directive

[WinCOFF] Add support for the .safeseh directive

.safeseh adds an entry to the .sxdata section to register all the
appropriate functions which may handle an exception.  This entry is not
a relocation to the symbol but instead the symbol table index of the
function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238641 91177308-0d34-0410-b5e6-96231b3b80d8