Diagnose error when the function args can't satisfy constexpr parameter requirements (#7269)

## Summary
This PR enhances constexpr validation by adding proper error checking
when function arguments cannot satisfy constexpr parameter requirements,
addressing issue #6370.

## Problem
Previously, when a function declared constexpr parameters, the compiler
would attempt to propagate constexpr-ness to the call site arguments,
but there was insufficient validation and error reporting when this
propagation failed. This could lead silent failures where constexpr
requirements weren't properly enforced

## Solution
This PR adds checks that:

1. **Validates constexpr arguments**: When a function parameter is
marked as `constexpr`, the compiler now explicitly checks that the
corresponding argument can be marked as `constexpr`

2. **Issues clear compilation errors**: added
`Diagnostics::argIsNotConstexpr`)

3. **Handles both call scenarios**: The validation works for both:
   - Direct function calls with IR-level function definitions
   - Calls to function from external modules

Fixes #6370

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Co-authored-by: slangbot <ellieh+slangbot@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

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