Clang emits `-Winteger-overflow` warnings from discarded `if constexpr`

[dangelog]

Testcase: https://gcc.godbolt.org/z/ds4M6zah4

#include <limits>

int main() {
    using Bounds = std::numeric_limits<long>;
    if constexpr (sizeof(long) < sizeof(long long)) {
        const long long longMaxPlusOne = static_cast<long long>(Bounds::max()) + 1;
        const long long longMinMinusOne = static_cast<long long>(Bounds::min()) - 1;
    }
}

On Unix x86-64, long has the same size of long long, so the if constexpr branch is discarded as the condition is false. However clang warns:

<source>:6:80: warning: overflow in expression; result is -9223372036854775808 with type 'long long' [-Winteger-overflow]
        const long long longMaxPlusOne = static_cast<long long>(Bounds::max()) + 1;
                                                                               ^
<source>:7:81: warning: overflow in expression; result is 9223372036854775807 with type 'long long' [-Winteger-overflow]
        const long long longMinMinusOne = static_cast<long long>(Bounds::min()) - 1;

It's a bit interesting that other kind of warnings are instead suppressed. For instance, if one adds a 123 / 0 outside of the if constexpr, Clang warns about the division by zero, but it doesn't if the division happens inside the if.

[shafik]

I believe this is because CheckedIntArithmetic(...) which is being used in these cases is using Info.Ctx.getDiagnostics().Report(...) as opposed to Info.CCEDiag(...) or similar.

[shafik]

Unfortunately just changing to use Info.CCEDiag(...) causes tests to break because we are using this diagnostic in non-constant expression contexts as well.

We need to know isDiscardedStatementContext() maybe we can modify Expr::EvaluateForOverflow(...) to take this as flag and modify EvalInfo to track this?

CC @zygoloid is there a better way?