[Bug c++/126978] New: ICE in push_namespace when parsing an invalid attribute on a namespace declaration

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126978

            Bug ID: 126978
           Summary: ICE in push_namespace when parsing an invalid
                    attribute on a namespace declaration
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: s.kimura.h41104 at gmail dot com
  Target Milestone: ---

Compiler Explorer: https://godbolt.org/z/aerxaKcMT

Reproducer:
```
foo(auto a);

namespace[[= 42]] N
```

Backtrace:
<source>:1:12: error: expected constructor, destructor, or type conversion
before ';' token
    1 | foo(auto a);
      |            ^
<source>:3:19: internal compiler error: in push_namespace, at
cp/name-lookup.cc:9630
    3 | namespace[[= 42]] N
      |                   ^
0x2beb468 diagnostics::context::diagnostic_impl(rich_location*,
diagnostics::metadata const*, diagnostics::option_id, char const*,
__va_list_tag (*) [1], diagnostics::kind)
        ???:0
0x2be00ab internal_error(char const*, ...)
        ???:0
0xb4805c fancy_abort(char const*, int, char const*)
        ???:0
0xd9e923 c_parse_file()
        ???:0
0xf31f99 c_common_parse_file()
        ???:0


This ICE goes back to gcc-16:
https://godbolt.org/z/7c9cc5G6E