[Enhancement] Diagnose constants with element type that is wider than necessary

clang has no diagnostic for constants whose element type is wider than necessary to hold all of the initializer values.

For example:

```c++
// All values fit in uint8_t, but the table is 4x-8x larger than it needs to be.
static const unsigned long lookup_table[256] = {
1, 2, 3, 4, 5, ...
};
```

This pattern can show up in code where a table was copied and pasted from another codebase.

The diagnostic can be implemented as either a clang warning or a clang-tidy check. If it's going to be implemented as a clang warning, it should be off by default, as the right type for the constant can be constrained by other factors (e.g., an external API).

Start by reviewing existing clang diagnostics and clang-tidy checks for constant arrays and initializer values; the issue names no specific files or tests. Decide whether this belongs as an opt-in clang warning or a clang-tidy check, then define coverage for arrays whose initializers fit a narrower element type and add tests showing the intended diagnostic behavior.

Written by the indexing model from the issue text.