[PATCH] Fix current stack unwinding when using DRASan

[Timur Iskhodzhanov]

  FTR, I do plan to fix the DynamoRIO part of DRASan to make GetCurrentThread() work too.
  However, I believe it's good to make this improvement to GET_STACK_TRACE_WITH_PC_AND_BP regardless.

http://llvm-reviews.chandlerc.com/D2290