self-hosted: improve handling of anonymous decls

 * anonymous decls have automatically generated names and symbols, and
   participate in the same memory management as named decls.
 * the Ref instruction is deleted
 * the DeclRef instruction now takes a `[]const u8` and DeclRefStr takes
   an arbitrary string instruction operand.
 * introduce a `zir.Decl` type for ZIR Module decls which holds
   content_hash and name - fields that are not needed for `zir.Inst`
   which are created as part of semantic analysis. This improves the
   function signatures of Module.zig and lowers memory usage.
 * the Str instruction is now defined to create an anonymous Decl and
   reference it.

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