Adds `AppendStructuredBuffer` and `ConsumeStructuredBuffer` definition
to HLSLExternalSemaSource. Adds separate tests for the AST shape and
element types, and adds constructor/handle.fromBinding test case to
shared test file for structured buffers.
These buffers do not have any subscript operators. Append and Consume
methods will be added later in llvm/llvm-project#112968.
Fixes#112777