Add mixin for Clang (#37)

Author: ChrisThrasher

clang.mixin

@@ -0,0 +1,5 @@
+build:
+  clang:
+    cmake-args:
+      - "-DCMAKE_C_COMPILER=clang"
+      - "-DCMAKE_CXX_COMPILER=clang++"

index.yaml

@@ -3,6 +3,7 @@ mixin:
   - build-testing.mixin
   - build-type.mixin
   - ccache.mixin
+  - clang.mixin
   - clang-libcxx.mixin
   - compile-commands.mixin
   - coverage.mixin