enabled after we'd found a few bugs that were caused by shadowed
local variables; the most important issue this turned up was
a common mistake of trying to obtain a mutex lock for the scope
of a code block by doing

        Mutex::Locker(m_map_mutex);

This doesn't assign the lock object to a local variable; it is
a temporary that has its dtor called immediately.  Instead,

        Mutex::Locker locker(m_map_mutex);

does what is intended.  For some reason -Wshadow happened to
highlight these as shadowed variables.

I also fixed a few obivous and easy shadowed variable issues
across the code base but there are a couple dozen more that
should be fixed when someone has a free minute.
<rdar://problem/12437585>

llvm-svn: 165269