LLVM 20.1.5

• Merged upstream v20.1.5.
• Disable libxml2 dependency for prebuilt packages. (15a392a)
• Mach-O: Support emitting the DWARF __debug_line section as non-debug section, in order to prevent the macOS linker from stripping it when linking the binary. This enables druntime's core.internal.backtrace to display file/line infos in backtraces without having to resort to an atos process. (356e996)