stage2: garbage collect unused anon decls

After this change, the frontend and backend cooperate to keep track of
which Decls are actually emitted into the machine code. When any backend
sees a `decl_ref` Value, it must mark the corresponding Decl `alive`
field to true.

This prevents unused comptime data from spilling into the output object
files. For example, if you do an `inline for` loop, previously, any
intermediate value calculations would have gone into the object file.
Now they are garbage collected immediately after the owner Decl has its
machine code generated.

In the frontend, when it is time to send a Decl to the linker, if it has
not been marked "alive" then it is deleted instead.

Additional improvements:
 * Resolve type ABI layouts after successful semantic analysis of a
   Decl. This is needed so that the backend has access to struct fields.
 * Sema: fix incorrect logic in resolveMaybeUndefVal. It should return
   "not comptime known" instead of a compile error for global variables.
 * `Value.pointerDeref` now returns `null` in the case that the pointer
   deref cannot happen at compile-time. This is true for global
   variables, for example. Another example is if a comptime known
   pointer has a hard coded address value.
 * Binary arithmetic sets the requireRuntimeBlock source location to the
   lhs_src or rhs_src as appropriate instead of on the operator node.
 * Fix LLVM codegen for slice_elem_val which had the wrong logic for
   when the operand was not a pointer.

As noted in the comment in the implementation of deleteUnusedDecl, a
future improvement will be to rework the frontend/linker interface to
remove the frontend's responsibility of calling allocateDeclIndexes.

I discovered some issues with the plan9 linker backend that are related
to this, and worked around them for now.

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