Use memoization for has()-matcher.

In TUs with large classes, a matcher like

  methodDecl(ofClass(recordDecl(has(varDecl()))))

(finding all member functions of classes with static variables)
becomes unbearably slow otherwise.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187115 91177308-0d34-0410-b5e6-96231b3b80d8