[improved-aliasing]: Reduce memory usage of memory tags

[Daniel Berlin]

This makes memory tags into their own type of DECL (inheriting from
tree_minimal_decl), reducing memory usage of the actual tag variables by
~70%. They used to be ~90 bytes per tag, now it's 36.  It also removes
the mem_tag_kind member of var_ann, and allows us to tell tags apart
from regular vars without the variable annotation.

Tags could be made smaller by splitting up decl_minimal, or making them
non-decl nodes entirely.  I have no plans to do either.

Bootstrapped and regtested on i686-pc-linux-gnu.
Applied to improved-aliasing-branch.


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