[LLVMdev] Final added to parser<bool>

[Rafael Espíndola]

> There are lots of valid reasons to want to be able to derive classes; that's
> why C++ lets you do that. :)

Note that LLVM is a toolchain toolkit. It is not trying to compete
with boost or the standard library.

To do its job LLVM has lots of auxiliary code, like DenseMap or the
option parsing, but there is absolutely no guarantees about their API,
ABI or even existence from one revision to the next.

Cheers,
Rafael