[libTooling] Add support for implicit `this` to `buildAddressOf`.

[ymandel]

Summary

Changes buildAddressOf to return this when given an implicit this expression.

clang/lib/Tooling/Transformer/SourceCodeBuilders.cpp

tooling::buildDereference(const Expr &E, const ASTContext &Context) {
   // Add leading '*'.
   if (needParensAfterUnaryOperator(E))
     return ("*(" + Text + ")").str();
   return ("*" + Text).str();
 }
 
 llvm::Optional<std::string> tooling::buildAddressOf(const Expr &E,
                                                     const ASTContext &Context) {
+  if (E.isImplicitCXXThis())
+    return std::string("this");
   if (const auto *Op = dyn_cast<UnaryOperator>(&E))
     if (Op->getOpcode() == UO_Deref) {
       // Strip leading '*'.
       StringRef Text =
           getText(*Op->getSubExpr()->IgnoreParenImpCasts(), Context);
       if (Text.empty())
         return llvm::None;
       return Text.str();

clang/unittests/Tooling/SourceCodeBuildersTest.cpp

 TEST(SourceCodeBuildersTest, BuildAddressOfPointerDereferenceIgnoresParens) {
   testBuilder(buildAddressOf, "S *x; *(x);", "x");
 }
 
 TEST(SourceCodeBuildersTest, BuildAddressOfBinaryOperation) {
   testBuilder(buildAddressOf, "S x; x + x;", "&(x + x)");
 }
 
+TEST(SourceCodeBuildersTest, BuildAddressOfImplicitThis) {
+  StringRef Snippet = R"cc(
+    struct Struct {
+      void foo() {}
+      void bar() {
+        foo();
+      }
+    };
+  )cc";
+  auto StmtMatch = matchStmt(
+      Snippet,
+      cxxMemberCallExpr(onImplicitObjectArgument(cxxThisExpr().bind("expr"))));
+  ASSERT_TRUE(StmtMatch);
+  EXPECT_THAT(buildAddressOf(*StmtMatch->Result.Nodes.getNodeAs<Expr>("expr"),
+                             *StmtMatch->Result.Context),
+              ValueIs(std::string("this")));
+}
+
 TEST(SourceCodeBuildersTest, BuildDereferencePointer) {
   testBuilder(buildDereference, "S *x; x;", "*x");
 }
 
 TEST(SourceCodeBuildersTest, BuildDereferenceValueAddress) {
   testBuilder(buildDereference, "S x; &x;", "x");
 }