Implicit resource bindings design

[bogner]

Once #176 is fleshed out, we need a design for implementing implicit resource bindings in Clang. We need to decide whether we are going to try to match DXC behavior or not, what information will be needed at various points in the compiler, and where will the implicit bindings and diagnostic take place.

There are two approaches to this:

1. Mark the bindings as "implicit" and calculate their actual locations later in the compiler
2. Fill out the bindings as we go.

Approach (2) would be a departure from DXC. There is some discussion of this starting in llvm/llvm-project#58051 (comment) but we need to discuss this in further detail if we want to pursue it. We would probably need to do some reachability analysis if we wanted to do this.

Some parts of approach (1) are probably necessary regardless of whether we adopt approach (2), because explicitly unbound resources exist in library shaders.