[Clang] Optimize -Wunsafe-buffer-usage. (#125492)

The Clang disgnostic `-Wunsafe-buffer-usage` was adding up to +15%
compilation time when used. Profiling showed that most of the overhead
comes from the use of ASTMatchers.

This change replaces the ASTMatcher infrastructure with simple matching
functions and keeps the functionality unchanged. It reduces the overhead
added by `-Wunsafe-buffer-usage` by 87.8%, leaving a negligible
additional compilation time of 1.7% when the diagnostic is used.

**Old version without -Wunsafe-buffer-usage:**
```
$ hyperfine -i -w 1 --runs 5 '/tmp/old_clang -c -std=c++20 /tmp/preprocessed.cc -ferror-limit=20' 
Benchmark 1: /tmp/old_clang -c -std=c++20 /tmp/preprocessed.cc -ferror-limit=20
  Time (mean ± σ):     231.035 s ±  3.210 s    [User: 229.134 s, System: 1.704 s]
  Range (min … max):   228.751 s … 236.682 s    5 runs
```

**Old version with -Wunsafe-buffer-usage:**
```
$ hyperfine -i -w 1 --runs 10 '/tmp/old_clang -c -std=c++20 /tmp/preprocessed.cc -ferror-limit=20 -Wunsafe-buffer-usage'
Benchmark 1: /tmp/old_clang -c -std=c++20 /tmp/preprocessed.cc -ferror-limit=20 -Wunsafe-buffer-usage
  Time (mean ± σ):     263.840 s ±  0.854 s    [User: 262.043 s, System: 1.575 s]
  Range (min … max):   262.442 s … 265.142 s    10 runs
```

**New version with -Wunsafe-buffer-usage:**
```
$ hyperfine -i -w 1 --runs 10 '/tmp/new_clang -c -std=c++20 /tmp/preprocessed.cc -ferror-limit=20 -Wunsafe-buffer-usage'
Benchmark 1: /tmp/new_clang -c -std=c++20 /tmp/preprocessed.cc -ferror-limit=20 -Wunsafe-buffer-usage
  Time (mean ± σ):     235.169 s ±  1.408 s    [User: 233.406 s, System: 1.561 s]
  Range (min … max):   232.221 s … 236.792 s    10 runs
```