[SPIR-V] Add proposal for vk::push_constant (#360)

Adds proposal to implement VK push constant support in Clang.

E2E implementation draft:
llvm/llvm-project#166793

Author: Keenuts

proposals/0035-spirv-push-constants.md

@@ -0,0 +1,90 @@
+---
+title: "[NNNN] - SPIR-V Push constants"
+params:
+  status: Design In Progress
+  authors:
+    - Keenuts: Nathan Gauër
+---
+
+## Introduction
+
+Push Constants are a Vulkan mechanism to pass a small amount of data to the
+shaders without creating buffers or modifying bindings.
+
+Usage is as follows:
+
+```hlsl
+struct S {
+  float3 light_color;
+};
+
+[[vk::push_constant]]
+S data;
+
+void main() {
+  [...]
+  do_something(data.light_color)
+}
+```
+## Motivation
+
+This is a core feature of HLSL+Vulkan.
+
+## Proposed Solution
+
+### HLSL - Parsing
+
+``[[vk::push_constant]]`` is a normal attribute in the `vk` namespace, with
+no custom parsing.
+This attribute is only allowed on global variables. Additional checks like
+storage class compatibility is done in sema.
+
+At this stage, we know the attribute can only be attached to a global
+variable.
+
+### HLSL - Sema
+
+During sema, compatibility checks between the global variable definition
+and the attribute are checked, and the global variable definition is created.
+
+Additional checks are:
+- The attribute is only allowed on a global variable with a struct type.
+- There can only be one ``[[vk::push_constant]]`` attribute in the shader.
+- There can be no VLA in the struct type (or a nested struct type).
+
+DXC allows any storage class to be attached to a push constant. I believe
+this is an oversight and we should probably refuse this. As result, the
+following rules will apply:
+
+- The variable storage class must be either `auto` or `uniform`.
+- The variable can be marked `const`, but is always considered `const`.
+
+### DXIL
+
+This being a Vulkan specific feature, the attribute is ignored when targeting
+DXIL. This means the push constant global becomes part of the cbuffer when
+building for DXIL.
+
+The attribute being ignored, the variable belong to the cbuffer, meaning no
+additional handling is required.
+
+### SPIR-V
+
+The created global variable after parsing has the following characteristics:
+  - the global variable will be marked as `const`.
+  - the variable address space will be `hlsl_push_constant`.
+
+Codegen will be left almost as-is as we simply load a variable in a different
+address space. The layout rule for those struct follows the same rules as
+[RW]StructuredBuffers.
+What we need to fixup is the cbuffer logic as a global variable with the
+HLSLVkPushConstant attribute should not be added to the constant buffer.
+
+The backend will then lower variables into the `hlsl_push_constant` address
+space into a `PushConstant` storage class. The rest is similar to any variable
+in another address space and should already be handled.
+
+## Draft PR
+
+There is also a draft PR implementing this end-to-end:
+ - https://github.com/llvm/llvm-project/pull/166793