Don't use unmangled name if property not found

This restores the previous behavior for this case. We'll continue
to use the mangled name, even if it does not correspond to a
declared

Don't use unmangled name if property not found

This restores the previous behavior for this case. We'll continue
to use the mangled name, even if it does not correspond to a
declared property.

This also fixes an assertion failure for the case of property
overwrite, as the add_new was not guaranteed to be "new" previously.

Fixes oss-fuzz #31045.

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