stage2: AstGen improvements

 * AstGen: represent compile errors in ZIR rather than returning
   `error.AnalysisFail`.
 * ZIR: remove decl_ref and decl_val instructions. These are replaced by
   `decl_ref_named` and `decl_val_named`, respectively, which will
   probably get renamed in the future to the instructions that were just
   deleted.
 * AstGen: implement `@This()`, `@fence()`, `@returnAddress()`, and
   `@src()`.
 * AstGen: struct_decl improved to support fields_len=0 but have decls.
 * AstGen: fix missing null bytes after compile error messages.
 * SrcLoc: no longer depend on `Decl`. Instead have an explicit field
   `parent_decl_node` which is an absolute AST Node index.
 * Module: `failed_files` table can have null value, in which case the
   key, which is a `*Scope.File`, will have ZIR errors in it.
 * ZIR: implement text rendering of struct decls.
 * CLI: introduce debug_usage and `zig astgen` command which is enabled
   when the compiler is built in debug mode.

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