This introduces the `captures` attribute as described in:
https://discourse.llvm.org/t/rfc-improvements-to-capture-tracking/81420
This initial patch only introduces the IR/bitcode support for the
attribute and its in-memory representation as `CaptureInfo`. This will
be followed by a patch to upgrade and remove the `nocapture` attribute,
and then by actual inference/analysis support.
Based on the RFC feedback, I've used a syntax similar to the `memory`
attribute, though the only "location" that can be specified is `ret`.
I've added some pretty extensive documentation to LangRef on the
semantics. One non-obvious bit here is that using ptrtoint will not
result in a "return-only" capture, even if the ptrtoint result is only
used in the return value. Without this requirement we wouldn't be able
to continue ordinary capture analysis on the return value.
- Move raw_ostream << operators for `ModRef` and `MemoryEffects` to a
new ModRef.cpp file under llvm/Support (instead of AliasAnalysis.cpp)
- This enables calling these operators from `Core` files like
Instructions.cpp (for instance for debugging). Currently, they live in
`LLVMAnalysis` which cannot be linked with `Core`.
