Make IPDBSession::getGlobalScope a non-const method

There doesn't seem to be a compelling reason why this method should be const
other than it was possible with the DIA implementation.  The native session
is going to act as a symbol factory and cache.  This could be acheived with
mutable (and the existing const_cast), but it seems cleaner to accept that
this method affects the state of the session.

This change eliminates an existing const_cast.

llvm-svn: 306041