[Sanitizer] Fix report_path functionality:

Summary:
  - Make sure mmap() is never called inside RawWrite function.
  - Wrap a bunch of standalone globals in a ReportFile object.
  - Make sure accesses to these globals are thread-safe.
  - Fix report_path functionality on Windows, where
    __sanitizer_set_report_path() would break program.

I've started this yak shaving in order to make
"CommonFlags::mmap_limit_mb" immutable. Currently we drop this flag
to zero before printing an error message.

Test Plan: regression test suite

Reviewers: kcc, glider

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6595