Add experimental Clang for P2561 (#97)

Author: vasama

build/build.sh

@@ -132,6 +132,12 @@ variadic-friends-trunk)
     URL=https://github.com/dancrn/llvm-project.git
     VERSION=variadic-friends-trunk-$(date +%Y%m%d)
     ;;
+p2561-trunk)
+    BRANCH=P2561
+    URL=https://github.com/vasama/llvm
+    VERSION=p2561-trunk-$(date +%Y%m%d)
+    LLVM_ENABLE_RUNTIMES+=";libunwind"
+    ;;
 bb-p2996-trunk)
     BRANCH=p2996
     URL=https://github.com/Bloomberg/clang-p2996