Remove -fseh-exceptions in favor of checking the triple

This option was misleading because it looked like it enabled the
language feature of SEH (__try / __except), when this option was really
controlling which EH personality function to use. Mingw only supports
SEH and SjLj EH on x86_64, so we can simply do away with this flag.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221963 91177308-0d34-0410-b5e6-96231b3b80d8
[Sanitizer] Refactor SanitizerArgs parsing in Driver.

Remove flag parsing details from the public header.
Use SanitizerSet to represent the set of enabled sanitizers.
Cleanup the implementation: update the comments to
reflect reality, remove dead code.

No functionality change.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221968 91177308-0d34-0410-b5e6-96231b3b80d8