Let RecursiveASTVisitor walk both syntactic and semantic form of InitListExprs.

Otherwise, this can lead to unexpected results when AST matching as
some nodes are only present in the semantic form.

For example, only looking at the syntactic form does not find the
DeclRefExpr to f() in:

  struct S { S(void (*a)()); };
  void f();
  S s[1] = {&f};

llvm-svn: 228138