Fix for two defects as follows:
- The IO mapping traverser was not setting inVisit, and would skip some AST nodes.
Depending on the order of nodes, this could have prevented the binding from
showing up in the generated SPIR-V.
- If a uniform array was flattened, each of the flattened scalars from the array
is still a (now-scalar) uniform. It was being converted to a temporary.
This PR factors out the code that knows how to walk just the live parts of the AST.
The traverser in reflect.cpp is renamed to TReflectionTraverser, and inherits from
TLiveTraverser, which will also be used by a future binding offset PR.
The code is now smart about the entry point name (no longer hardcoded to "main").
There is an option to traverse all code (live+dead), because a consumer of the
class may wish to use it for both purposes without wanting a whole separate
class hierarchy.
