No warning for implicit narrowing conversion during constant evaluation

[katzdm]

The fixes for C++23 P2564 merged by #89565 expanded the class of constant evaluated variable initializers. This inadvertently resulted in some warnings no longer diagnosing, e.g.,

constexpr char h = u'\u5678';  // previously diagnosed warn_impcast_integer_precision_constant

A suggested approach is to mark both diagnoses of warn_impcast_integer_precision_constant as DiagRuntimeBehavior (which was previously the case), and to separately diagnose in the constant evaluator. This could be done by deducing narrowing during evaluation, or by marking narrowing expressions during semantic analysis and thereafter diagnosing if any such expressions are evaluated. Ideally, the above would diagnose, but an initializer like the following would not:

constexpr char h = (true ? 'a' : u'\u5678');