[llvm] Remove unused standard headers: <string>, <optional>, <numeric>, <tuple> (PR #167232)

[Kazu Hirata via llvm-commits]

================
@@ -54,7 +54,6 @@
 #include <cassert>
 #include <cstring>
 #include <optional>
-#include <string>
----------------
kazutakahirata wrote:

Looking at the build errors, we still need this for `hash_value`?

```
template <typename T>
hash_code hash_value(const std::basic_string<T> &arg) {
  return hash_combine_range(arg);
}
```


https://github.com/llvm/llvm-project/pull/167232