[HLSL] Investigate how to handle erroring/warning of out-of-bounds vector access

[Icohedron]

Currently, Clang does not emit a warning or error upon encountering an out-of-bounds vector index.
This does not match DXC behavior, which emits an error

https://godbolt.org/z/3Mfjvn5Ex

export float test(float4 v4) {
    return v4[5];
}
/* DXC output
<source>:2:15: error: vector element index '5' is out of bounds
    return v4[5];
              ^
*/

https://godbolt.org/z/zGc9ox5d1

export float2 test(float4 v4) {
    float2 res = { v4[4], v4[5] };
    return res;
}
/* DXC output
error: validation errors
<source>:3:5: error: Instructions should not read uninitialized value.
note: at 'ret <2 x float> undef' in block '#0' of function '�?test@@YA?AV?$vector@M$01@@V?$vector@M$03@@@Z'.
Validation failed.
*/

Should we match DXC behavior and error upon encountering an out-of-bounds vector index? Or should we simply create a warning?
Should we error/warn only for HLSL, or for all languages? Perhaps error for HLSL but warn for all other languages?

An error for out-of-bounds vector indexing has attempted to be implemented before (#128952), but it broke builds (see #128952 (comment)).

We should also consider what @efriedma-quic mentioned (#128952 (comment)) about generally not wanting to have differing semantic rules based on whether or not a value is constant.