Revert r301735 (and subsequent r301786).

It leads to clang crashing, e.g. on this short code fragment (added to
test/SemaCXX/warn-thread-safety-parsing.cpp):

  class SomeClass {
  public:
    void foo() {
      auto l = [this] { auto l = [] EXCLUSIVE_LOCKS_REQUIRED(mu_) {}; };
    }
    Mutex mu_;
  };

llvm-svn: 301916