[clang] [Clang] Cache stable DeclContext relationships (NFC) (PR #223047)

[Mehdi Amini via cfe-commits]

joker-eph wrote:

Yes, the "about" page confirms it: https://llvm-compile-time-tracker.com/about.php

```
Reproducing

The compile time tracker tests the CTMark portion of the [LLVM test-suite](https://llvm.org/docs/TestSuiteGuide.html) against specific cached CMake configurations. You can set up a test-suite build as follows:

git clone https://github.com/llvm/llvm-test-suite.git
mkdir build
cd build
cmake .. -G Ninja \
    -C ../cmake/caches/$CONFIG.cmake \
    -DCMAKE_C_COMPILER=$PATH_TO_CLANG
The $CONFIGs used by the tracker are O3, ReleaseThinLTO, ReleaseLTO-g and O0-g.
```

I asked my fork to be added so that I can submit branches for measurement there, but my local before/after is meant to reproduce the same thing.

https://github.com/llvm/llvm-project/pull/223047