Rollup of 7 pull requests

[Centril]

Successful merges:

• Point at opaque and closure type definitions in type errors #66463 (Point at opaque and closure type definitions in type errors)
• Reduce special treatment for zsts #67501 (Reduce special treatment for zsts)
• Parse the syntax described in RFC 2632 #67820 (Parse the syntax described in RFC 2632)
• rustc_ast_lowering: misc cleanup & rustc dep reductions #67922 (rustc_ast_lowering: misc cleanup & rustc dep reductions)
• Allow specifying LLVM args in target specifications #68059 (Allow specifying LLVM args in target specifications)
• Extend support of _ in type parameters #68071 (Extend support of _ in type parameters)
• expect fn after const unsafe / const extern #68073 (expect fn after const unsafe / const extern)

Failed merges:

r? @ghost