[HLSL][Matrix] Failure to initalize vector via cast of a matrix to vector

[farzonl]

The code below is failing in clang/lib/Sema/SemaInit.cpp via a FK_ConversionFailed in clang when it works fine in DXC.

https://godbolt.org/z/r9G7fYzcj

export float4 fn(float2x2 m) {
    float4 v = m;
    return v;
}

Note the explicit cast version crashes today, but wont after: #168915

export float4 fn(float2x2 m) {
    float4 v = (float4)m;
    return v;
}

error: cannot initialize a variable of type 'float4' (aka 'vector<float, 4>') with an rvalue of type 'float2x2' (aka 'matrix<float, 2, 2>')
    2 |     float4 v = (float4)m;
      |            ^   ~~~~~~~~~~~

The bug is happening because we are not hitting this block in InitializationSequence::InitializeFrom

// For HLSL ext vector types we allow list initialization behavior for C++
  // functional cast expressions which look like constructor syntax. This is
  // accomplished by converting initialization arguments to InitListExpr.
  if (S.getLangOpts().HLSL && Args.size() > 1 &&
      (DestType->isExtVectorType() || DestType->isConstantMatrixType()) &&
      (SourceType.isNull() ||
       !Context.hasSameUnqualifiedType(SourceType, DestType))) {
    InitListExpr *ILE = new (Context)
        InitListExpr(S.getASTContext(), Args.front()->getBeginLoc(), Args,
                     Args.back()->getEndLoc());
    ILE->setType(DestType);
    Args[0] = ILE;
    TryListInitialization(S, Entity, Kind, ILE, *this,
                          TreatUnavailableAsInvalid);
    return;
  }

We can see if we did every thing would work fine:
https://godbolt.org/z/GMej3Kvvr

export float4 fn(float2x2 m) {
    float4 v = {m};
    return v;
}

Ie if we did vector initalization via matrix inside an initalizer list everthing would work just fine.