Eliminate CDHashMap::insertAtContextLevelZero (#8173)

Eliminate CDHashMap::insertAtContextLevelZero (#8173)

This method unnecessarily complicates the usage of CDHashMap. It had one usage in the current code, for dealing with global declarations. However, this was an entirely artificial use case, as one should properly manage scopes when this option is true (i.e. global-declarations simply disables user-level scoping in the symbol table).

It also simplifies the symbol table so that it doesn't automatically push a global outermost scope. Instead, this scope is pushed when the logic is declared, so that background symbols are correctly added at level 0.

Fixes #4767.