`Wmissing-prototypes` and `Wmissing-variable-declarations` should not trigger in a module

The following translation unit

```c++
export module m;

void f() {
}

int x;
```

when compiled with `clang++ -std=c++20 -Wmissing-prototypes -Wmissing-variable-declarations` gives a warning of

```console
:3:6: warning: no previous prototype for function 'f' [-Wmissing-prototypes]
3 | void f() {
| ^
:3:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
3 | void f() {
| ^
| static
:6:5: warning: no previous extern declaration for non-static variable 'x' [-Wmissing-variable-declarations]
6 | int x;
| ^
:6:1: note: declare 'static' if the variable is not intended to be used outside of this translation unit
6 | int x;
| ^
2 warnings generated.
Compiler returned: 0
```

The purpose of these warnings is to prevent violations of the one-definition rule. Modules already prevent this problem because the function is defined in the module purview, so no warning is needed in these cases.

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

Reproduce the warning with clang++ using the C++20 module translation unit and both warning flags shown in the issue. Trace how diagnostics are emitted for declarations in a module purview; done means f and x no longer trigger these warnings there while the warnings remain applicable in non-module translation units.

Written by the indexing model from the issue text.