swift234

When instantiating a generic class, compensate for
unexpected forematter from the superclass.

This requires a pretty substantial shift in the
generic-metadata allocation/initialization dance
because (1) we can't allocate class metadata without
knowing what the superclass is and (2) the offset
from the metadata cache entry to the address point is
no longer determined solely by the metadata pattern.

While I'm making invasive changes to metadata, fix
two race conditions in metadata creation.  The first
is that we need to ensure that only one thread succeeds
at lazily creating a generic-metadata cache.  The second
is that we need to ensure that only one thread actually
attempts to create a particular metadata; any others
should block until the metadata is successfully built.

This commit finishes rdar://17776354.  LLDB will
need to adjust to the runtime-private metadata layout
changes.

Swift SVN r20537