[llvm-exegesis] Add error handling for fork failures (#65186)

There are still some transient failures on the clang-avx512 builder on
the new subprocess memory tests. Some of them seem to be related to an
inability to fork, but it's hard to debug currently as there is no
explicit error handling for a failed fork call, and nice error reporting
for a failed fork is something that we should have regardless.

Author: boomanaiden154

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

@@ -263,6 +263,12 @@ class SubProcessFunctionExecutorImpl
       return AddMemDefError;
 
     pid_t ParentOrChildPID = fork();
+
+    if (ParentOrChildPID == -1) {
+      return make_error<Failure>("Failed to create child process: " +
+                                 Twine(strerror(errno)));
+    }
+
     if (ParentOrChildPID == 0) {
       // We are in the child process, close the write end of the pipe
       close(PipeFiles[1]);