[llvm] r179285 - Add a function to check if an argument list is too long.

[Rafael Espíndola]

> Please include unittests for things under Support where possible. This one
> seems quite easy.

How? The interesting part of this function is its connection to the OS
(the call to sysconf or the value copied from the windows doc). We can
stub that, but then the test is just that the function adds the size
of each argument.

That is why my suggestion is to just test that the clang driver works
when given a large command line. It might work because the system it
is running in has a really large limit (current linux for example) or
because it is creating an @file.

Cheers,
Rafael