Fix llvm-extract's "writing bitcode to a terminal" warning, which wasn't

working. To support this, add an is_displayed() function to raw_ostream,
and generalize Process::StandardOutIsDisplayed and friends in order to
support it.

Also, call RemoveFileOnSignal before creating a file instead of after, so
that the file isn't left behind if the program is interrupted between when
the file is created and RemoveFileOnSignal is called.

While here, add a -S to llvm-extract and port it to IRReader so that it
supports assembly input.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81568 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/System/Process.h

@@ -94,6 +94,11 @@ namespace sys {
       /// the user rather than being put on a pipe or stored in a file.
       static bool StandardErrIsDisplayed();
 
+      /// This function determines if the given file descriptor is connected to
+      /// a "tty" or "console" window. That is, the output would be displayed to
+      /// the user rather than being put on a pipe or stored in a file.
+      static bool FileDescriptorIsDisplayed(int fd);
+
       /// This function determines the number of columns in the window
       /// if standard output is connected to a "tty" or "console"
       /// window. If standard output is not connected to a tty or