### Prefetch Symbol Resolution
Based on this
[suggestion](https://discourse.llvm.org/t/rfc-code-prefetch-insertion/88668/29?u=rlavaee),
we must identify if a prefetch target is defined in the current module
to avoid **undefined symbol errors**. Since this occurs during
sequential **CodeGen**, we must rely on function names rather than IR
Module APIs.
**Key Changes:**
* **`MachineFunction` Integration:** Added a `PrefetchTargets` field
(with serialization) to track all targets associated with a function.
* **Guaranteed Emission:** All prefetch targets are now emitted
regardless of basic block or callsite index matches to ensure the symbol
exists.
* **Fallback Placement:** Targets with non-matching callsite indices are
emitted at the end of the block to resolve the reference.
This is the first PR to enable the prefetch optimization via Propeller
based on our
[RFC](https://discourse.llvm.org/t/rfc-code-prefetch-insertion/88668/22).
It enables emitting special symbols prefixed with
`__llvm_prefetch_target` to point to the prefetch targets as specified
via directives in the profile. A prefetch target is uniquely identified
by its function name, basic block ID, and the subblock index (used when
the target is after a call instruction).
A new pass is added which sets a field in basic blocks which have
prefetch targets. The next PR will add the prefetch insertion logic into
the same pass.
