Absl containers don't have a free swap method

### Describe the issue

A typical idiom from the [copy & swap idiom](https://stackoverflow.com/a/3279550) is to do sth like this:
```c++
class MyClass {
int i;
std::unordered_set std_set;
absl::flat_hash_set absl_set;

// ...
friend void swap(MyClass& m1, MyClass& m2) noexcept {
using std::swap; // default
swap(m1.i, m2.i); // std::swap(T&,T&)
swap(m1.std_set, m2.std_set); // std::swap(std::unordered&, std::unordered&)
swap(m1.absl_set, m2.absl_set); // std::swap(T&, T&) instead of m1.swap(m2)
}

};
```

But Absl containers don't have a free swap method. So the code above picks the default `std::swap(T&, T&)` implementation instead of using `m2.swap(m2)`.

Adding sth like this resolves the issue:
```c++
template
class flat_hash_set {
// ...

// will be found due to ADL
friend void swap(flat_hash_set& set2) noexcept {
set2.swap(set2);
}
};
```

### Steps to reproduce the problem

see above

### What version of Abseil are you using?

lts_2023_08_02

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

Linux zen 6.5.11-1-MANJARO #1 SMP PREEMPT_DYNAMIC Thu Nov 9 02:34:53 UTC 2023 x86_64 GNU/Linux

### What compiler and version are you using?

```
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --enable-languages=ada,c,c++,d,fortran,go,lto,objc,obj-c++ --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --with-build-config=bootstrap-lto --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-libstdcxx-backtrace --enable-link-serialization=1 --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.1 20230801 (GCC)
```

### What build system are you using?

cmake version 3.27.7

### Additional context

_No response_