[Attributor] Use a pointer value type for the access kind -> accesses map

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: 616219 (381559/s)
temporary memory allocations: 83294 (51575/s)
peak heap memory consumption: 72.15MB
peak RSS (including heaptrack overhead): 160.04MB
total memory leaked: 269.04KB
```

After:
```
calls to allocation functions: 595004 (357145/s)
temporary memory allocations: 83840 (50324/s)
peak heap memory consumption: 52.14MB
peak RSS (including heaptrack overhead): 138.32MB
total memory leaked: 269.04KB
```

Difference:
```
calls to allocation functions: -21215 (-415980/s)
temporary memory allocations: 546 (10705/s)
peak heap memory consumption: -20.01MB
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

---