[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.

A issue aponta para um problema de desempenho em tempo de compilação nos headers de Cord e flat_hash_map do Abseil. Comece examinando a cadeia de dependências dos headers, especialmente como o hasher de flat_hash_map inclui Cord. Analise a estrutura do código-fonte do Abseil para entender as relações entre os includes. Reproduza o build lento usando a flag clang -ftime-trace fornecida e o Bazel. Uma correção provavelmente envolve refatorar as dependências dos headers para evitar forçar a compilação de Cord para usuários que não usam Cord.

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