[clang] Inconsistent attributes for dllexport __cxxabiv1:: __fundamental_type_info with -fvisibility=hidden

Fundamentally, hidden visibility (`-fvisibility=hidden`) and dllexport are mutually exclusive. But in practice this is not an issue, dllexport is stronger and overrides `-fvisibility=hidden`.

```c++
void __declspec(dllexport) func(void) { }
void noexport(void) { }
```
```console
$ clang -target x86_64-windows-gnu -S -emit-llvm -o - export.cpp -fvisibility=hidden
[...]
define dso_local dllexport void @_Z4funcv() #0 {
entry:
ret void
}

[...]
define hidden void @_Z8noexportv() #0 {
entry:
ret void
}
```

If we explicitly try to specify the incompatible combination, we get an error:
```c++
void __declspec(dllexport) __attribute__((visibility("hidden"))) func(void) { }
```
```console
$ clang -target x86_64-windows-gnu -S -emit-llvm -o - export-hidden.cpp
export-hidden.cpp:1:66: error: hidden visibility cannot be applied to
'dllexport' declaration
1 | void __declspec(dllexport) __attribute__((visibility("hidden"))) func...
| ^
1 error generated.
```

However, for the type `__cxxabiv1:: __fundamental_type_info`, Clang implicitly generates symbols, where this can end up in a conflict:
```c++
namespace __cxxabiv1 {
struct
#ifdef EXPORT
__attribute__((dllexport))
#endif
__fundamental_type_info {
virtual ~__fundamental_type_info();
};
__fundamental_type_info::~__fundamental_type_info() {}
} // namespace __cxxabiv1
```
```console
$ clang -target x86_64-windows-gnu repro.cpp -fvisibility=hidden -c
$ clang -target x86_64-windows-gnu repro.cpp -fvisibility=hidden -c -DEXPORT
dllexport GlobalValue must have default or protected visibility
ptr @_ZTIv
dllexport GlobalValue must have default or protected visibility
ptr @_ZTSv
dllexport GlobalValue must have default or protected visibility
ptr @_ZTIPv
dllexport GlobalValue must have default or protected visibility
ptr @_ZTSPv
dllexport GlobalValue must have default or protected visibility
ptr @_ZTIPKv
dllexport GlobalValue must have default or protected visibility
ptr @_ZTSPKv
[...]
fatal error: error in backend: Broken module found, compilation aborted!
```

Start by compiling the repro.cpp example with and without -DEXPORT using the two clang commands shown in the issue. Trace the implicitly generated __cxxabiv1:: fundamental_type_info symbols and how -fvisibility=hidden combines with dllexport. Done means the reproducer no longer produces a broken module or fatal backend error, with the intended visibility behavior preserved.

Written by the indexing model from the issue text.