warn-unused-ignores doesn't fire when ignoring multiple codes #11157

To Reproduce

# test.py 

a = object()

def f() -> int:
    return a[b'c'] # type: ignore[index, override]

Expected Behavior

a is not indexable but ignored.
An error along the lines of unused "type: ignore" comment for override

Actual Behavior

% mypy --warn-unused-ignores test.py 
Success: no issues found in 1 source file

Your Environment

% mypy --version
mypy 0.910