[Attributor][NFC] Replace the nested AAMap with a key pair

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: 512375 (362871/s)
temporary memory allocations: 98746 (69933/s)
peak heap memory consumption: 22.54MB
peak RSS (including heaptrack overhead): 106.78MB
total memory leaked: 269.10KB
```

After:
```
calls to allocation functions: 509833 (338534/s)
temporary memory allocations: 98902 (65671/s)
peak heap memory consumption: 18.71MB
peak RSS (including heaptrack overhead): 103.00MB
total memory leaked: 269.10KB
```

Difference:
```
calls to allocation functions: -2542 (-27042/s)
temporary memory allocations: 156 (1659/s)
peak heap memory consumption: -3.83MB
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B
```