[libc++] Refactor __split_buffer to eliminate code duplication (#114138)

This PR refactors the `__split_buffer` class to eliminate code
duplication in the `push_back` and `push_front` member functions.

**Motivation:**  
The lvalue and rvalue reference overloads of `push_back` share identical
logic, which coincides with that of `emplace_back`. Similarly, the
overloads of `push_front` also share identical logic but lack an
`emplace_front` member function, leading to an inconsistency. These
identical internal logics lead to significant code duplication, making
future maintenance more difficult.

**Summary of Refactor:**  
This PR reduces code redundancy by:
1. Modifying both overloads of `push_back` to call `emplace_back`.
2. Introducing a new `emplace_front` member function that encapsulates
the logic of `push_front`, allowing both overloads of `push_front` to
call it (The addition of `emplace_front` also avoids the inconsistency
regarding the absence of `emplace_front`).

The refactoring results in reduced code duplication, improved
maintainability, and enhanced readability.

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