Define __guard_{iat,longjmp}_{count,table} symbols.

VC2017 contains these new symbols as undefined symobls. They are used
for /guard:cf. Since we do not support the control flow guard, but we
want to at least ignore these symbols so that we can link against VS2017
libraries.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=727193.

llvm-svn: 305876