"External declaration [decl name] is unused" after resource assignment

Description
Given a global extern resource with an auto-assigned or user assigned resource register, reassigning the variable to a different resource (e.g. from ResourceDescriptorHeap) results in the "external declaration is unused" validation error, even if the resource is used prior to assignment.

Steps to Reproduce

CE link

RWByteAddressBuffer buffer : register(u0);

[numthreads(8, 8, 1)]
void CSMain(uint3 id : SV_DispatchThreadID)
{
        buffer.Store(id.x, 0);
        buffer = ResourceDescriptorHeap[0];
        buffer.Store(id.x, 0);
}

Actual Behavior

error: validation errors
<source>:1: error: External declaration '\01?buffer@@3URWByteAddressBuffer@@A' is unused.
Validation failed.

Note that this may be intended behavior (understandably), however, I think in this case the error message is misleading and should directly indicate that this sort of reassignment is forbidden if so.

Environment

• DXC version 1.7.2207
• Host Operating System Windows

Start with the HLSL reproducer in the CE link and inspect the compiler validation path for a global extern resource that is used before reassignment. Determine whether the reassignment should be accepted; done means either correcting the misleading unused-declaration diagnostic or reporting clearly that this reassignment is forbidden, with coverage for the reproducer.