[Driver] Stop passing -fseh-exceptions for x86_64-windows-msvc

-fseh-exceptions is only meaningful for MinGW targets, and that driver
already has logic to pass either -fdwarf-exceptions or -fseh-exceptions
as appropriate. -fseh-exceptions is just a no-op for MSVC triples, and
passing it to cc1 causes unnecessary confusion.

Differential Revision: https://reviews.llvm.org/D47850

llvm-svn: 334145