Drop support for legacy PM with LLVM 15
LLVM 15 already removes some of the legacy PM APIs we're using. This patch forces use of NewPM with LLVM 15 (with `-Z new-llvm-pass-manager=no` throwing a warning) and stubs out various FFI methods with a report_fatal_error on LLVM 15.
For LLVMPassManagerBuilderPopulateLTOPassManager() I went with adding our own wrapper, as the alternative would be to muck about with weak symbols, which seems to be non-trivial as far as cross-platform support is concerned (std has `weak!` for this purpose, but only as an internal utility.)
Fixes#96072.
Fixes#96362.