[PECOFF] Driver support for /SAFESEH option.

Syntactically /SAFESEH is a boolean flag -- you can pass /SAFESEH or /SAFESEH:no.

The meaning of /SAFESEH is as follows.

 - If /SAFESEH is specified, the linker will produce an executable with SEH table.
   If any input files are not compatible with SEH, it's an error.
 - If /SAFESEH:no is specified, the linker will not emit SEH table even if all
   input files are compatible with SEH.
 - If no option is specified, the linker emits SEH table if all input files are
   compatible with SEH.

llvm-svn: 201895