Use a lambda for calls to ::open in RetryAfterSignal

In Bionic, open can be overloaded for _FORTIFY_SOURCE support, causing
compile errors of RetryAfterSignal due to overload resolution. Wrapping
the call in a lambda avoids this.

Based on a patch by Chih-Wei Huang <cwhuang@linux.org.tw>!

llvm-svn: 340751