[Support] Add ExitOnError utility to support tools that use the exit-on-error

[Support] Add ExitOnError utility to support tools that use the exit-on-error
idiom.

Most LLVM tool code exits immediately when an error is encountered and prints an
error message to stderr. The ExitOnError class supports this by providing two
call operators - one for Errors, and one for Expected<T>s. Calls to code that
can return Errors (or Expected<T>s) can use these calls to bail out on error,
and otherwise continue as if the operation had succeeded. E.g.

Error foo();
Expected<int> bar();

int main(int argc, char *argv[]) {
  ExitOnError ExitOnErr;

  ExitOnErr.setBanner(std::string("Error in ") + argv[0] + ":");

  // Exit if foo returns an error. No need to manually check error return.
  ExitOnErr(foo());

  // Exit if bar returns an error, otherwise unwrap the contained int and
  // continue.
  int X = ExitOnErr(bar());

  // ...

  return 0;
}

llvm-svn: 263749