[sanitizer] Restore stderr when using forkpty() to spawn external symbolizer

In `AtosSymbolizer`, we're using `forkpty()` to create a new pseudo-terminal to communicate with the `atos` tool (we need that to avoid output buffering in interactive mode). This however redirects both stdout and stderr into a single stream, so when we read the output, we can't distinguish between errors and standard replies. Let's save&restore stderr to avoid that.

Differential Revision: http://reviews.llvm.org/D15073

llvm-svn: 265923