[clang] [HLSL] Add matrix constructors using initalizer lists (PR #162743)

[Chris B via cfe-commits]

https://github.com/llvm-beanz commented:

Your testing is focused on `CXXFunctionalCastExpr`, but the implementation of that should be the same as the general initializer list implementation (it is for vectors), so the testing also needs to cover those cases.

Things like:

```
[numthreads(1,1,1)]
void main() {
  float4x2 m = {1.xxx, 2.xx, 3.x, 4.xx};
  struct S { float2x2 x; float2x2 y;};
  S s = {m};
  float2x2 m2 = {0.xxxx};
  int4x4 m3 = {m2, m2, m2, m2};
}
```

https://godbolt.org/z/WaK51nWTb

https://github.com/llvm/llvm-project/pull/162743