Properly handle the mismatching "mutating" signature for implementing interfaces

[jkwak-work]

Problem description
When the interface declares a function with [mutating], the implementation also has to have [mutating].
If the implementation didn't have the keyword, Slang crashes with an internal error.

Goal
We should print a proper error for the mistake.
Or it should work even when the implementation didn't have the matching [mutating] keyword.

Repro steps
The following code can reproduce the issue.

//TEST:SIMPLE:
//TEST:SIMPLE:-DWITH_MUTATING
interface IHitInfo
{
};

interface IClosestHitQuery
{
    [mutating] void traceRay(out IHitInfo hit);
};

struct GeneratePathClosestHitQuery : IClosestHitQuery
{
#if defined(WITH_MUTATING)
    [mutating]
#endif
    void traceRay(out IHitInfo hit)
    {
        hit = {};
    }
};

Note that the modifier out is required to reproduce the issue.

[tomas-davidovic]

Also, note that this only crashes when the out parameter is itself an interface. Normal types (pods and structs) work.