Previously imemo_ast was handled as WB-protected which caused a segfault
of the following code:
# shareable_constant_value: literal
M0 = {}
M1 = {}
...
M100000 = {}
My analysis is here: `shareable_constant_value: literal` creates many
Hash instances during parsing, and add them to node_buffer of imemo_ast.
However, the contents are missed because imemo_ast is incorrectly
WB-protected.
This changeset makes imemo_ast as WB-unprotected.
2c7d3b3a72