[Bug]: Cord causes slow builds all over project

### Describe the issue

While diagnosing my slow builds in a project that depends on Abseil, I concluded that Cord is the outstanding culprit. I don't use Cord, but I do use flags, and flags depend on flat_hash_map, and flat_hash_map comes with a header that _can_ hash a Cord, and thereby depends on Cord. It would be neat if there was a way to untie this knot so that only Cord users depended on Cord and its hasher.

According to `-ftime-trace` analysis these are the top three most expensive template instantiations for a small portion of my build that really should have been trivial but actually takes 55s to compile:

```
**** Templates that took longest to instantiate:
3932 ms: std::deque::operator= (31 times, avg 126 ms)
3861 ms: std::deque::assign::__assign_wi... (31 times, avg 123 ms)
```

### Steps to reproduce the problem

Use clang's `-ftime-trace` flag in your project that needs Abseil-cpp flags, then analyze the output with ClangBuildAnalyzer.

### What version of Abseil are you using?

HEAD

### What operating system and version are you using?

Ubuntu Linux 24.04

### What compiler and version are you using?

/opt/llvm-19/bin/clang -v
clang version 19.1.7
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/llvm-19/bin
Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/11
Selected GCC installation: /usr/lib/gcc/aarch64-linux-gnu/11
Candidate multilib: .;@m64
Selected multilib: .;@m64

### What build system are you using?

Bazel 7.6.1

### Additional context

These results are seen with libc++, if that makes a difference.

L'issue pointe vers un problème de performances à la compilation dans les headers de Cord et flat_hash_map d'Abseil. Commencez par examiner la chaîne de dépendances des headers, en particulier la manière dont le hasher de flat_hash_map inclut Cord. Examinez la structure du code source d'Abseil pour comprendre les relations d'inclusion. Reproduisez le build lent à l'aide du flag clang -ftime-trace fourni et de Bazel. Une solution implique probablement de refactoriser les dépendances des headers afin d'éviter d'imposer la compilation de Cord aux utilisateurs qui n'utilisent pas Cord.

Rédigé par le modèle d'indexation à partir du texte de l'issue.