transpile: prune_unwanted_decls only after setting prenamed_decls

[Rua]

• Fixes transpile: typedef of anonymous types are lost if name is not used #1738

[fw-immunant]

Panic in CI:

[ FAILED ] translate enum_fwd_decl.c
thread 'main' panicked at 'Could not find CDeclId(155) in TypedAstContext', c2rust-transpile/src/c_ast/mod.rs:1009:44
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d394408fb38c4de61f765a3ed5189d2731a1da91/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/d394408fb38c4de61f765a3ed5189d2731a1da91/library/core/src/panicking.rs:142:14
   2: c2rust_transpile::c_ast::TypedAstContext::set_prenamed_decls
   3: c2rust_transpile::translator::translate
   4: c2rust_transpile::transpile_single
   5: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
   6: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
   7: c2rust_transpile::transpile
   8: c2rust_transpile::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[Rua]

Yeah, it's panicking on the problem reported in #1740. The AST contains a CDeclId that doesn't refer to anything.