Handle llvm.launder.invariant.group in msan.

Summary:
[MSan] handle llvm.launder.invariant.group

    Msan used to give false-positives in

    class Foo {
     public:
      virtual ~Foo() {};
    };

    // Return true iff *x is set.
    bool f1(void **x, bool flag);

    Foo* f() {
      void *p;
      bool found;
      found = f1(&p,flag);
      if (found) {
        // p is always set here.
        return static_cast<Foo*>(p); // False positive here.
      }
      return nullptr;
    }

Patch by Ilya Tokar.

Reviewers: #sanitizers, eugenis

Reviewed By: #sanitizers, eugenis

Subscribers: eugenis, Prazek, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68236

llvm-svn: 373515