Update Linux build instructions to use Clang-18 with libc++ and latest TDLib

[Copilot]

This PR updates the Linux build script (linux-ubuntu-18.sh) to use modern build tooling and the latest version of TDLib, improving compatibility and performance.

Key Changes

Compiler Migration: Switched from GCC (g++) to Clang-18 with libc++ standard library for better C++ standards compliance and performance:

• Added clang-18, libc++-18-dev, and libc++abi-18-dev packages
• Updated both cmake invocations to use CXXFLAGS="-stdlib=libc++" CC=/usr/bin/clang-18 CXX=/usr/bin/clang++-18

TDLib Version: Removed the pinned v1.6.0 checkout to use the latest TDLib version, ensuring access to newest features and bug fixes.

Source Processing: Added proper TDLib source preparation steps:

• php SplitSource.php before building to prepare the source tree
• php SplitSource.php --undo after building to clean up

Package Updates: Changed php to php-cli for more explicit dependency management.

Build Process

The updated script now follows the recommended TDLib build process:

1. Install modern build dependencies including Clang-18
2. Clone latest TDLib source
3. Prepare source with SplitSource.php
4. Build TDLib with Clang-18 and libc++
5. Build Java bindings with same toolchain
6. Clean up source modifications

This ensures compatibility with the latest TDLib API changes and provides a more robust build using modern C++ tooling.

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