Allow unsafe operations in unsafe functions in generated code.

This will soon be denied by default, but we trust that what cxx does is
correct.

Bug: 290018030
Test: m rust
Change-Id: Icde1116d90c068c7ca14c60f5b03f3fca99561f6
Merged-In: If38e434a5d9e3320cfa7e6f686a078f7ee764435