[IR] Introduce captures attribute (#116990)

This introduces the `captures` attribute as described in:
https://discourse.llvm.org/t/rfc-improvements-to-capture-tracking/81420

This initial patch o

[IR] Introduce captures attribute (#116990)

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.

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[NFC] Fix line endings for ModRef.cpp/OptionStrCmp.cpp (#109712)

[NFC][Support] Eliminate ',' at end of MemoryEffects print (#106545)

- Eliminate comma at end of a MemoryEffects print.
- Added basic unit test to validate that.

[NFC][Support] Move ModRef/MemoryEffects printers to their own file (#105367)

- Move raw_ostream << operators for `ModRef` and `MemoryEffects` to a
new ModRef.cpp file under llvm/Support (instead o

[NFC][Support] Move ModRef/MemoryEffects printers to their own file (#105367)

- 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`.

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