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

---------

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

1 files changed, 6 insertions, 3 deletions

diff --git a/tests/diagnostics/constexpr-error.slang b/tests/diagnostics/constexpr-error.slang
index 0a4744b71..a79c71847 100644
--- a/tests/diagnostics/constexpr-error.slang
+++ b/tests/diagnostics/constexpr-error.slang
@@ -24,7 +24,7 @@ float4 main() : SV_Target
     result += t.Sample(s, uv, int2(0,0));
 
     // Error: data passed through cbuffer isn't compile-time constant
-    // CHECK: ([[# @LINE+1]]): error 40006:
+    // CHECK: ([[# @LINE+1]]): error 40012:
     result += t.Sample(s, uv, offset);
 
     // Error: data computed via conditional isn't compile-time cosntant
@@ -33,16 +33,19 @@ float4 main() : SV_Target
     {
     	ii = 1;
     }
-    // CHECK: ([[# @LINE+1]]): error 40006:
+    // CHECK: ([[# @LINE+1]]): error 40012:
     result += t.Sample(s, uv, int2(ii));
 
     // Error: data computed in loop isn't compile-time constant
     // (and loop isn't unroll-able)
-    // CHECK: ([[# @LINE+1]]): error 40006:
+    // CHECK: ([[# @LINE+1]]): error 40012:
     for(uint jj = 0; jj < uv.y; jj++)
     {
     	result += t.Sample(s, uv, int2(jj));
     }
 
+
+
+
     return result;
 }
\ No newline at end of file