Make c++ defer looks like a swift one

[przybylski]

This code uses some c++ trickery to make defer macro looks almost like defer keyword in swift.

Instad of:

defer([&]() {
  /* cleanup code */
});

coders can now drop some boilerplate and do this:

defer {
  /* cleanup code */
};

But this changes capture list to all values by reference

[nadavrot]

@przybylski , please find the author of this code (using git blame) and invite them to review this code (by using their github handle name)

[przybylski]

@lattner could you have a look on this PR?

[jckarter]

I prefer the function to macro trickery, honestly. Neat trick though.

[lattner]

Simultaneously cute and horrible :-) - looks great, thanks!