[LibTooling] Add Explanation parameter to `makeRule`.

Summary:
Conceptually, a single-case RewriteRule has a matcher, edit(s) and an (optional)
explanation. `makeRule` previously only took the matcher and edit(s). This
change adds (optional) support for the explanation.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62390

llvm-svn: 361643
diff --git a/clang/unittests/Tooling/TransformerTest.cpp b/clang/unittests/Tooling/TransformerTest.cpp
index 0bf012a..41c7e7a 100644
--- a/clang/unittests/Tooling/TransformerTest.cpp
+++ b/clang/unittests/Tooling/TransformerTest.cpp
@@ -147,8 +147,7 @@
                                   on(expr(hasType(isOrPointsTo(StringType)))
                                          .bind(StringExpr)),
                                   callee(cxxMethodDecl(hasName("c_str")))))),
-      change(text("REPLACED")));
-  R.Cases[0].Explanation = text("Use size() method directly on string.");
+      change(text("REPLACED")), text("Use size() method directly on string."));
   return R;
 }