[libclang]: visit C++17 if init statements

[milianw]

Summary

This makes the previously unaccessible AST nodes for C++17 "if with
init statements" accessible to consumers of libclang.

[milianw]

ping?

[bkramer]

looks good

clang/test/Index/cxx17-if-with-initializer.cpp

+// Test is line- and column-sensitive; see below.
+
+void foo() {
+  if (bool bar = true; bar) {
+  }
+}
+
+// RUN: c-index-test -test-load-source all -std=c++17 %s | FileCheck -check-prefix=CHECK-LOAD %s
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:3:6: FunctionDecl=foo:3:6 (Definition) Extent=[3:1 - 6:2]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:3:12: CompoundStmt= Extent=[3:12 - 6:2]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:4:3: IfStmt= Extent=[4:3 - 5:4]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:4:7: DeclStmt= Extent=[4:7 - 4:23]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:4:12: VarDecl=bar:4:12 (Definition) Extent=[4:7 - 4:22]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:4:18: CXXBoolLiteralExpr= Extent=[4:18 - 4:22]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:4:24: UnexposedExpr=bar:4:12 Extent=[4:24 - 4:27]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:4:24: DeclRefExpr=bar:4:12 Extent=[4:24 - 4:27]
+// CHECK-LOAD: cxx17-if-with-initializer.cpp:4:29: CompoundStmt= Extent=[4:29 - 5:4]

clang/tools/libclang/CIndex.cpp

 }
 void EnqueueVisitor::VisitGotoStmt(const GotoStmt *GS) {
   WL.push_back(LabelRefVisit(GS->getLabel(), GS->getLabelLoc(), Parent));
 }
 void EnqueueVisitor::VisitIfStmt(const IfStmt *If) {
   AddStmt(If->getElse());
   AddStmt(If->getThen());
   AddStmt(If->getCond());
+  AddStmt(If->getInit());
   AddDecl(If->getConditionVariable());
 }
 void EnqueueVisitor::VisitInitListExpr(const InitListExpr *IE) {
   // We care about the syntactic form of the initializer list, only.
   if (InitListExpr *Syntactic = IE->getSyntacticForm())
     IE = Syntactic;
   EnqueueChildren(IE);
 }