Move detection of writes and shadowing of __debug__ from compiler to symtable.

[iritkatriel]

There are checks in the compiler for writes and shadowing of __debug__. This can be done earlier, in the symtable, where it is somewhat simpler.

Linked PRs

• gh-122245: move checks for writes and shadowing of __debug__ to symtable #122246
• gh-122245: Add test case of generic type with __debug__ #122322