build: allow using the old toolchain on the rebranched LLVM

Author: compnerd

utils/build-windows-rebranch.bat

@@ -176,6 +176,7 @@ cmake^
     -DLLVM_ENABLE_OCAMLDOC:BOOL=NO^
     -DLLVM_ENABLE_LIBXML2:BOOL=NO^
     -DLLVM_ENABLE_ZLIB:BOOL=NO^
+    -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON^
     -DENABLE_X86_RELAX_RELOCATIONS:BOOL=YES^
     -DLLVM_INSTALL_BINUTILS_SYMLINKS:BOOL=YES^
     -DLLVM_INSTALL_TOOLCHAIN_ONLY:BOOL=YES^