Closes Homebrew/homebrew#20239.

This replaces the old behaviour of falling back to the original
compiler with no messaging.

Fixes Homebrew/homebrew#19170.
Fixes mistydemeo/tigerbrew#45.

Build 211 can build most things, but I've seen intermittent
miscompilation and slower code. Also it comes from Xcode 4.2, so most
people don't have it anyway. Let's use 218 as the minimum viable
version.

The existing case-statement with nested if-statements is gross and hard
to extend. Replacing it with a priority queue simplifies the logic and
makes it very easy to add new compilers to the fails_with system, which
we will likely want to do in the future.

The original constraints that led to using a custom collection rather
than Array or Set here no longer exist, so let's avoid the pointless
abstraction here.

Given the current state of OS X compilers, the original fails_with
behavior is becoming less useful, mostly resulting in build failures
each time the compiler is updated. So make the following changes:

When a build is specified, we retain the old behavior: switch compilers
if the available compiler is <= the build, don't switch if it is > the
build.

When no build is specified, unconditionally switch compilers, and don't
output the advice message. This allows us to mark formulae as
perpetually failing, avoiding the need to update formulae each time a
new compiler build is made available.

As a bonus, this makes the logic much easier to reason about.

Closes Homebrew/homebrew#18175.

Actually works properly now.

This reverts commit a0e56345ea55f7ee3c41d6bf13f9cfb3af635323.

This reverts commit c98d50495275ff4951dd126bb88a55e568b64092.

Closes Homebrew/homebrew#17618.
Closes Homebrew/homebrew#17813.

Inheriting from Array (and other core types) is problematic:

  - It exposes a very wide interface with many methods that are not
    really relevant to the subclass.
  - It can cause some weird side effects, as many Array operations are
    in C and have hardcoded return values; for example, combining two
    array subclasses returns a new Array instead of the subclass.

Avoid these problems using delegation and the Enumerable module where
applicable.

Fixes Homebrew/homebrew#14121.

 - Formulae can now declare failures on any compiler.
 - FailsWithLLVM and associated formula elements have been moved to
   compat.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>