DWARF Simplified Template Names: Narrow down the handling for operator overloads

Actually we can, for now, remove the explicit "operator" handling
entirely - since clang currently won't try to flag any of these as
rebuildable. That seems like a reasonable state for now, but it could be
narrowed down to only apply to conversion operators, most likely - but
would need more nuance for op> and op>> since they would be incorrectly
flagged as already having their template arguments (due to the trailing
'>').