Vanilla LLVM 20.1.4 +

• LLD linker, including a tiny workaround for LDC integration. (e9e9f10)
• compiler-rt libraries, including some tweaks for the builtins library on Windows and Android. Use -DCOMPILER_RT_INCLUDE_TESTS=OFF in the CMake command line to avoid the dependency on clang. (9531854, 133dae7)
• Experimental Xtensa target.
• BOLT tools and libraries (Linux & Android packages only).
• Windows: Use . instead of :: as CodeView debuginfo scope separator (for fully qualified names) for modules tagged with the D language, i.e., if compiling with LDC and -g. (5317ba5)
• Windows: Fix PGO. (c6f3c3b)
• Automated prebuilt package generation via CI. (da08766)