[libc++] Remove <string> and <vector> includes from <flat_map> (#114876)

`<string>` doesn't seem to be required at all and `flat_map` doesn't
support `vector<bool>`, so we can include just `vector<T>`. This cuts
the include time in half on my system.

GitOrigin-RevId: 332fda86fb20c6c2cdc58976a8739c6a13110734
8 files changed