Drop support for legacy PM with LLVM 15

[nikic]

Support for the legacy pass manager is being actively removed in LLVM 15. We need to make -Z new-llvm-pass-manager=no error with LLVM 15 (or warn and ignore), and remove references to certain FFI functions either exported by the Rust wrapper or llvm-c.

This would be at least the LLVMRustCreate*Pass family of functions, as well as the LLVMPsasManagerBuilder* family of functions. As the latter are provided by llvm-c, I guess we'd want to use weak symbols for those.