Implement feature for disabling opaque pointers on LLVM 15 and 16

[Derppening]

Description

This pull request implements support for using typed pointers in LLVM 15. This is implemented by several steps:

• Add feature typed-pointers to distinguish APIs for typed vs opaque pointers
• Implicitly set the use of typed pointers during Context creation
• Add typed-pointers feature dependency on all LLVM versions that only have typed pointer support (i.e. LLVM 4-14)
• Updated #[cfg(...)] blocks to use feature = "typed-pointers" instead of version guards for pointer-related APIs
• Add version guards in lib.rs to ensure that the typed-pointers feature can only be enabled in LLVM 15-16

Note that previous defaults are still present - LLVM 15 and 16 still uses opaque pointers by default, and will only switch to typed pointers if the typed-pointers feature is enabled.

Some points of note or for discussion with regards to the current implementation:

• The current implementation uses a Cargo feature to guard typed vs opaque pointer API usage. Are there any known use-cases where a user of this library requires both?
  • If so, how should we handle APIs that have previously been version-guarded? E.g. Builder::build_load, where the typed pointer version of this API calls LLVMBuildLoad while the opaque pointer version calls LLVMBuildLoad2.
• LLVM technically supports both typed and opaque pointers from LLVM 13-16 15-16, although only LLVM 15 officially supports both. Should I also expand the scope of this PR to include opt-in into opaque pointers in LLVM 13-14, as well as opt-out out of opaque pointers in LLVM 16?
  • If so, how should this look like?
  • Edit: I just realized that LLVMContextSetOpaquePointers is only available in LLVM 15-16, so please disregard my message regarding LLVM 13-14.

Related Issue

#489

How This Has Been Tested

This PR is tested using:

$ cargo test --workspace --features llvm15-0-prefer-dynamic,no-libffi-linking
$ cargo test --workspace --features llvm15-0-prefer-dynamic,no-libffi-linking,typed-pointers
$ cargo test --workspace --features llvm16-0-prefer-dynamic,no-libffi-linking
$ cargo test --workspace --features llvm16-0-prefer-dynamic,no-libffi-linking,typed-pointers

AFAICT all tests (including doctests) pass with LLVM 15 and 16 under both opaque pointer and typed pointer modes.

Checklist

• I have read the Contributing Guide

[Derppening]

v2: Added the ability to disable opaque pointers in LLVM 16 as well. PR description is also updated to reflect this fact.

[TheDan64]

Ahh, adding typed-pointers as a feature flag is a neat idea!