[Attributor] Use a pointer value type for the AAMap

This reduces memory consumption and the need to copy complex data
structures repeatedly.

No functional change is intended.

---

Single run of the Attributor module and then CGSCC pass (oldPM)
for SPASS/clause.c (~10k LLVM-IR loc):

Before:
```
calls to allocation functions: 613353 (376521/s)
temporary memory allocations: 83636 (51341/s)
peak heap memory consumption: 75.64MB
peak RSS (including heaptrack overhead): 162.97MB
total memory leaked: 269.04KB
```

After:
```
calls to allocation functions: 616575 (349929/s)
temporary memory allocations: 83650 (47474/s)
peak heap memory consumption: 72.15MB
peak RSS (including heaptrack overhead): 159.81MB
total memory leaked: 269.04KB
```

Difference:
```
calls to allocation functions: 3222 (24225/s)
temporary memory allocations: 14 (105/s)
peak heap memory consumption: -3.49MB
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

---