[HLSL] Add an attribute to differentiate resource attributes for Buffer vs StructuredBuffer and RawBuffer

[bogner]

The target extension types for buffers are split into dx.TypedBuffer and dx.RawBuffer in order to separate the types of operation that are possible on the two. Specifically, Typed buffers must be accessed one typed element at a time, whereas Raw and Structured buffers don't have this restriction.

We need to represent this difference in the HLSL AST in order to correctly generate the target types. To do so, we should introduce a type attribute [[raw_buffer]] that will be used on buffer types such as StructuredBuffer or ByteAddressBuffer that do not require typed element access.
 
Note: This is related to llvm/wg-hlsl#68 and llvm/wg-hlsl#42. We should add this attribute to the table in that document.

AC:

• A new type attribute is introduced that conveys this information on an __hlsl_resource_t

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Justin Bogner (bogner)

The target extension types for buffers are split into [dx.TypedBuffer and dx.RawBuffer]( https://llvm.org/docs/DirectX/DXILResources.html#buffers) in order to separate the types of operation that are possible on the two. Specifically, Typed buffers must be accessed one typed element at a time, whereas Raw and Structured buffers don't have this restriction.

We need to represent this difference in the HLSL AST in order to correctly generate the target types. To do so, we should introduce a type attribute [[raw_buffer]] that will be used on buffer types such as StructuredBuffer or ByteAddressBuffer that do not require typed element access.
 
Note: This is related to llvm/wg-hlsl#68 and llvm/wg-hlsl#42. We should add this attribute to the table in that document.

AC:

• A new type attribute is introduced that conveys this information on an __hlsl_resource_t