[PATCH] Clear LD_PRELOAD when forking external symbolizer.

[Sergey Matveev]

  I couldn't find anything related to setenv in bash sources, but I reverted to a custom implementation anyway.

  > Can we do a "custom" implementation by dlsym("unsetenv") lookup (similar to current SetEnv implementation)?
  No. It's not safe to call dlsym after a fork.

http://llvm-reviews.chandlerc.com/D704

BRANCH
  ld_preload_in_symbolizer

ARCANIST PROJECT
  compiler-rt