[MS Demangler] Support extern "C" functions.

There are two cases we need to support with extern "C"
functions.  The first is the case of a '9' indicating that
the function has no prototype.  This occurs when we mangle
a symbol inside of an extern "C" function, but not the
function itself.

The second case is when we have an overloaded extern "C"
functions.  In this case we emit $$J0 to indicate this.
This patch adds support for both of these cases.

llvm-svn: 339471