stage2: free Sema's arena after generating machine code

Previously, linker backends or machine code backends were able to hold
on to references to inside Sema's temporary arena. However there can
be large objects stored there that we want to free after machine code is
generated.

The primary change in this commit is to use a temporary arena for Sema
of function bodies that gets freed after machine code backend finishes
handling `updateFunc` (at the same time that Air and Liveness get freed).

The other changes in this commit are fixing issues that fell out from
the primary change.

 * The C linker backend is rewritten to handle updateDecl and updateFunc
   separately. Also, all Decl updates get access to typedefs and
   fwd_decls, not only functions.
 * The C linker backend is updated to the new API that does not depend
   on allocateDeclIndexes and does not have to handle garbage collected
   decls.
 * The C linker backend uses an arena for Type/Value objects that
   `typedefs` references. These can be garbage collected every so often
   after flush(), however that garbage collection code is not
   implemented at this time. It will be pretty simple, just allocate a
   new arena, copy all the Type objects to it, update the keys of the
   hash map, free the old arena.
 * Sema: fix a handful of instances of not copying Type/Value objects
   from the temporary arena into the appropriate Decl arena.
 * Type: fix some function types not reporting hasCodeGenBits()
   correctly.

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