stage2: improve source locations of Decl access

 * zir.Code: introduce a decls array. This is so that `decl_val` and
   `decl_ref` instructions can refer to a Decl with a u32 and therefore
   they can also store a source location. This is needed for proper
   compile error reporting.
 * astgen uses a hash map to avoid redundantly adding a Decl to the
   decls array.
 * fixed reporting "instruction illegal outside function body" instead
   of the desired message "unable to resolve comptime value".
 * astgen skips emitting dbg_stmt instructions in comptime scopes.
 * astgen has some logic to avoid adding unnecessary type coercion
   instructions for common values.

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