clang-cl: Map /EHs- to -fno-exceptions

This isn't 100% compatible with MSVC, but it's close enough.  MSVC's /EH
flag doesn't really control exceptions so much as how to clean up after
an exception is thrown.  The upshot is that cl.exe /EHs- will compile
try, throw, and catch statements with a warning, but clang-cl will
reject such constructs with a hard error.  We can't compile such EH
constructs anyway, but this may matter to consumers of the AST.

Reviewers: hans

Differential Revision: http://reviews.llvm.org/D4317