[analyzer] Properly invalidate global regions on opaque function calls.

This fixes a regression where a call to a function we can't reason about
would not actually invalidate global regions that had explicit bindings.

  void test_that_now_works() {
    globalInt = 42;
    clang_analyzer_eval(globalInt == 42); // expected-warning{{TRUE}}

    invalidateGlobals();
    clang_analyzer_eval(globalInt == 42); // expected-warning{{UNKNOWN}}
  }

This has probably been around since the initial "cluster" refactoring of
RegionStore, if not longer.

<rdar://problem/13464044>

llvm-svn: 179553