[MS ABI] Don't rely on terminatepad

We'd like to remove support for terminatepad from LLVM.  To do this, we
need to move Clang off of it first.  The intent behind terminatepad was
to carefully model exception specifications for the MSVC personality.

However, we don't support exception specifications for the MSVC
personality and neither does MSVC.  Instead, MSVC supports
all-or-nothing exception specifications.  We can model this limited
usage using cleanuppads which call std::terminate.

Differential Revision: http://reviews.llvm.org/D15478

llvm-svn: 255521