…ons (shader-slang#8113)

When using Slang reflection API to find functions by name in a type,
`FunctionReflection::getName()` would return `nullptr` for overloaded
functions instead of the expected function name.

The issue occurred in `spReflectionFunction_GetName` when
`findFunctionByNameInType` returned a `SlangReflectionFunction` wrapping
an `OverloadedExpr` (for overloaded functions) instead of a single
`DeclRef<FunctionDeclBase>`. The `convertToFunc()` function would fail
for `OverloadedExpr` objects, causing `getName()` to return `nullptr`.

**Example of the bug:**
```cpp
// This code would fail before the fix
public interface IBase {
    public void step(inout float f);
}

public struct Impl : IBase {
    public void step(inout float f) { f += 1.0f; }
}

// Using reflection API:
auto implType = reflection->findTypeByName("Impl");
auto stepFunction = reflection->findFunctionByNameInType(implType, "step");
auto name = stepFunction->getName(); // Would return nullptr
```

**Fix:**
Modified `spReflectionFunction_GetName` to handle overloaded functions
by falling back to the name of the first overload candidate when
`convertToFunc` fails. This follows the same pattern already used by
`spReflectionFunction_specializeWithArgTypes`.

The fix ensures that:
- Overloaded function containers return the correct function name
- Individual overload candidates also return their names correctly  
- Non-overloaded functions continue to work as before
- No regression in existing functionality

**Testing:**
Added comprehensive test cases covering both the original issue scenario
and explicit function overloading. All existing reflection tests
continue to pass.

Fixes shader-slang#8047.

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