HLSL initializer list support

[llvm-beanz]

HLSL initializer lists are... odd...

Basically HLSL flattens any structures or vector arguments to an InitListExpr, and flattens the shape of the target variable. As long as they then agree on how many elements need to be assigned, the assignment is valid.

The following are valid HLSL assignments:

struct A {
  float X;
  float Y;
};
struct B {
  A Arr[2];
  int Z;
};

A ah = {1.0, 2.0};
float2 F2 = {1.0, 2.0};
A aah = {F2};
B bah = { {1, 2}, {3, 4}, 5};
B bahh = {1,2,3,4,5};
B bahhh = {F2, 1,2,3};
B bahhhh = {{F2}, {F2}, 3};
B bahhhhh = {1,2,3,F2};

And more...

We want to deprecate this syntax in the future, however we believe that there is enough existing code that depends on this that we will need to have clang support it.

AC:

• clang supports DXC-style flattened initializer lists

[damyanp]

If we accept https://github.com/microsoft/hlsl-specs/blob/main/proposals/0005-strict-initializer-lists.md then we don't need to do this.

Note follow ups for that proposal:

• clang emits fixits/warnings when it detects code relying on this, with the plan being that a future language version doesn't accept it
• refactoring pattern added to allow clangd to update code

[damyanp]

Set to medium priority since we're targeting building a collection of shaders that rely on this behavior, but we also have the option of updating these shaders.

[llvm-beanz]

Assigning this to myself to start working on the language for the spec.

[spall]

https://godbolt.org/z/bhYeMYsq4
Array Initialization that doesn't currently work.