[Sanitizer] Improve external symbolizer behavior.

1) Don't start external symbolizer subprocess until we actually try to
   symbolize anything.
2) Allow to turn off external symbolizer by providing empty ?SAN_SYMBOLIZER_PATH
   environment variable.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@195771 91177308-0d34-0410-b5e6-96231b3b80d8