Rollup merge of #132319 - Zalathar:add-module-flag, r=jieyouxu

cg_llvm: Clean up FFI calls for setting module flags

This is a combination of several inter-related changes to how module flags are set:

- Remove some unnecessary code for setting an `"LTOPostLink"` flag, which has been obsolete since LLVM 17.
- Define our own enum instead of relying on enum values defined by LLVM's unstable C++ API.
- Use safe wrapper functions to set module flags, instead of direct `unsafe` calls.
- Consistently pass pointer/length strings instead of C strings.
- Remove or shrink some `unsafe` blocks.

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