Don't overwrite unrelated variables with for-loop-variables

for-loops that were not inside a function could overwrite global
and universal variables with the loop variable.  Avoid this by making
for-loop-variables local variables in their enclosing scope.

This means that if someone does:

    set a global
    for a in local; end
    echo $a

The local $a will shadow the global one (but not be visible in child
scopes). Which is surprising, but less dangerous than the previous
behavior.

The detection whether the loop is running inside a function was failing
inside command substitutions. Remove this special handling of functions
alltogether, it's not needed anymore.

Fixes #6480

src/parse_execution.h

@@ -66,7 +66,6 @@ class parse_execution_context_t {
     wcstring get_source(const parse_node_t &node) const;
     tnode_t<grammar::plain_statement> infinite_recursive_statement_in_job_list(
         tnode_t<grammar::job_list> job_list, wcstring *out_func_name) const;
-    bool is_function_context() const;
 
     // Expand a command which may contain variables, producing an expand command and possibly
     // arguments. Prints an error message on error.