[RFC] Replace `%` with `$` in variable

[SchrodingerZhu]

I directly utilize the SSA operand display name as the name appearing in debug information. While this provides a direct association in debugging experience, dereference or other operations becomes impossible in GDB/LLDB TUI as %operand->xxx cannot be parsed easily.

Ultimately, when tmp_var is merged in upstream, we should not use this SSA name implicitly.

1. Provide a pass to associate gccjit.var_name attribute with each SSA value if a user really want to debug with SSA ids. This pass should have an option to control mangling behavior such as replacing % with $ in variable.
2. Maintain var_name properly during lowering
3. Make all other irrelevant intermediate values as unnamed tmp vars.