[clang][modules] Cannot include and import `string` with libstdc++

The following two translation units

module;

#include <string>

export module b;

namespace std {

export using std::string;

}

#include <string>

import b;

Cause clang to fail to compile with

Step cmake returned: 0
-- The CXX compiler identification is Clang 24.0.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: /app/build
Step build returned: 1
[1/6] Scanning /app/b.cpp for CXX dependencies
[2/6] Scanning /app/c.cpp for CXX dependencies
[3/6] Generating CXX dyndep file CMakeFiles/lib.dir/CXX.dd
[4/6] Building CXX object CMakeFiles/lib.dir/b.cpp.o
[5/6] Building CXX object CMakeFiles/lib.dir/c.cpp.o
FAILED: CMakeFiles/lib.dir/c.cpp.o 
/opt/compiler-explorer/clang-assertions-trunk/bin/clang++ --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot   -fcolor-diagnostics -fno-crash-diagnostics -g -std=c++26 -MD -MT CMakeFiles/lib.dir/c.cpp.o -MF CMakeFiles/lib.dir/c.cpp.o.d @CMakeFiles/lib.dir/c.cpp.o.modmap -o CMakeFiles/lib.dir/c.cpp.o -c /app/c.cpp
In module 'b' imported from /app/c.cpp:3:
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/17.0.0/../../../../include/c++/17.0.0/x86_64-linux-gnu/bits/c++config.h:377:43: error: cannot add 'abi_tag' attribute in a redeclaration
  377 |   inline namespace __cxx11 __attribute__((__abi_tag__ ("cxx11"))) { }
      |                                           ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/17.0.0/../../../../include/c++/17.0.0/bits/basic_string.h:4563:1: note: previous declaration is here
 4563 | _GLIBCXX_BEGIN_NAMESPACE_CXX11
      | ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/17.0.0/../../../../include/c++/17.0.0/x86_64-linux-gnu/bits/c++config.h:384:51: note: expanded from macro '_GLIBCXX_BEGIN_NAMESPACE_CXX11'
  384 | # define _GLIBCXX_BEGIN_NAMESPACE_CXX11 namespace __cxx11 {
      |                                                   ^
1 error generated.
ninja: build stopped: subcommand failed.

See it live: https://godbolt.org/z/zaY14ET7x

This reduces down to

namespace a {

inline namespace ai __attribute__((__abi_tag__ ("a"))) { }

}

namespace a {
inline namespace ai {

void af();

}
}

module;

#include "a.hpp"

export module b;

namespace a {

export using a::af;

}

#include "a.hpp"

import b;

void c() {
	a::af();
}

See it live: https://godbolt.org/z/s1en115EY

This is a somewhat recent regression, but I'm not at a computer fast enough for me to do the git bisect to find which commit introduced it. This fails on trunk but passes on clang 23.

Start with the reduced C++ reproducer involving an abi_tagged inline namespace, then compare its behavior on Clang 23 and trunk. Trace the module import and redeclaration handling that produces the abi_tag error, and add or update a regression test showing that the reproducer compiles successfully.