Attribute on namespace overrides `-fvisibility-inlines-hidden`

Compile the following code with `-fPIC -fvisibility-inlines-hidden`:

```c++
__attribute__((visibility("default")))
inline int id(int x) { return x; }

struct __attribute__((visibility("default"))) S {
static int id(int x) { return x; }
};

namespace N __attribute__((visibility("default"))) {
inline int id(int x) { return x; }
}

int test(int x) {
return id(x) + S::id(x) + N::id(x);
}
```

We emit only `.hidden S::id(int)`, but GCC additionally emits `.hidden N::id(int)`.

Technically, the [GCC docs](https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#index-fvisibility-inlines-hidden) don't talk about namespaces:

> You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

However:
* Namespaces create a weaker connection than classes, so if an attribute on classes doesn't override the flag, an attribute on namespaces should also not do it.
* The motivation talks about "a particular inline method", so it seems the intention is to only take attributes on the function itself into account.
* We're diverging from GCC here.

This can be observed when building with libstdc++ and `-fsemantic-interposition -fvisibility-inlines-hidden`: the library applies default visibility on namespace level, which prevents inlining of inline functions because they get default visibility and thus have to be interposable.

Start by compiling the supplied C++ reproducer with -fPIC -fvisibility-inlines-hidden under LLVM and GCC, then inspect the emitted visibility for S::id(int) and N::id(int). The issue names no source files or tests; done means resolving whether namespace attributes should affect inline visibility and aligning LLVM's behavior with the documented intent and GCC output.

Written by the indexing model from the issue text.