Support: Add a utility to remap std{in,out,err} to /dev/null if closed

It's possible to start a program with one (or all) of the standard file
descriptors closed.  Subsequent open system calls will give the program
a low-numbered file descriptor.

This is problematic because we may believe we are writing to standard
out instead of a file.

Introduce Process::FixupStandardFileDescriptors, a helper function to
remap standard file descriptors to /dev/null if they were closed before
the program started.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219170 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Process.h

@@ -186,6 +186,12 @@ public:
                     ArrayRef<const char *> ArgsFromMain,
                     SpecificBumpPtrAllocator<char> &ArgAllocator);
 
+  // This functions ensures that the standard file descriptors (input, output,
+  // and error) are properly mapped to a file descriptor before we use any of
+  // them.  This should only be called by standalone programs, library
+  // components should not call this.
+  static std::error_code FixupStandardFileDescriptors();
+
   /// This function determines if the standard input is connected directly
   /// to a user's input (keyboard probably), rather than coming from a file
   /// or pipe.