Unwrap MIDL-synthesised anonymous enum names into top-level constants

[Copilot]

MIDL assigns synthetic tag names (e.g. __MIDL___MIDL_itf_dia2_0000_0000_0001) to enumerations that were anonymous in the original IDL. The clang parser was treating these as named enums instead of unwrapping their variants as top-level constants.

• cx.rs: Added is_midl_anonymous_enum_name() — returns true for names starting with __MIDL_, the prefix MIDL uses for all synthesised anonymous-type names.
• mod.rs: Extended the anonymous-enum check to also cover MIDL-synthesised names, so their variants are emitted as top-level RDL constants (same path as truly unnamed enum { ... }).
• roundtrip/midl_enum.{h,rdl}: New roundtrip golden test exercising the fix.

Before:

#[repr(i32)]
enum __MIDL___MIDL_itf_test_0000_0000_0001 {
    E_PDB_OUT_OF_MEMORY = -2140340221,
}

After:

const E_PDB_OUT_OF_MEMORY: i32 = -2140340221;