[llvm] Metadata: Optimize metadata queries (PR #70700)

[Matthias Braun via llvm-commits]

MatzeB wrote:

In my ad-hoc benchmarking I am measuring `clang -O3 -c llvm-test-suite/CTMark/sqlite3` compile-time as instructions counted by valgrind/callgrind:

```
Before e3cf80c              37,821,687,947
After 3cf80c                38,133,312,923
this change before e3cf80c  37,268,867,198
this change after 3cf80c    37,268,867,064
```


https://github.com/llvm/llvm-project/pull/70700