[compiler-rt] Fix broken (flaky) unittests based on FlagParser.

Summary:
The FlagParser is populating a static global class with the
unrecognized flags when parsing. That global class has a 
dcheck that limit the number of unrecognized flag to 20.

```
class UnknownFlags {
  static const int kMaxUnknownFlags = 20;
  const char *unknown_flags_[kMaxUnknownFlags];
  int n_unknown_flags_;

  [...]

  void Report() {
    if (!n_unknown_flags_) return;
    Printf("WARNING: found %d unrecognized flag(s):\n", n_unknown_flags_);
    for (int i = 0; i < n_unknown_flags_; ++i)
      Printf("    %s\n", unknown_flags_[i]);
    n_unknown_flags_ = 0;
  }
};

UnknownFlags unknown_flags;
```

Unittests based on that class must reset the counter 'n_unknown_flags_' or
the next usage of that class may fail arbitrary. This can be done by
reporting the pending unknown flags.

Reviewers: rnk

Subscribers: llvm-commits, wang0109, kubabrecka, chrisha

Differential Revision: http://reviews.llvm.org/D21896

llvm-svn: 274234