Fixes Homebrew/homebrew#27420.

Instead check if the path exists by using opt.

After recent cleanups the code in this branch should be unreachable, so
there is little point in keeping it around.

These methods are no longer required.

This reflects what Apple calls it in more recent versions of Xcode/OS X.

Since ae177adb2bd55ee5ad6367e7639c4cf0c774b63a, we can safely assume
that xcrun works, and a functioning xcrun will search dev_tools_path and
xctoolchain_path, so we can stop doing extra work here.

On CLT-only 10.7 and 10.8, xcrun will not work, but all the tools will
be in /usr/bin, which we check before invoking xcrun. Further, in this
case, dev_tools_path will be /usr/bin, and xctoolchain_path will not
exist, so the fallbacks here are unnecessary.

It is activated by the same mechanism as the Homebrew/versions compilers
which now check if the GCC formula uses the same, correct version.

References Homebrew/homebrew#28418.

This is handled here:
https://github.com/Homebrew/homebrew/blob/67fbb313da8843234c4d1785c9c074af5a025394/Library/brew.rb#L34-46

Closes Homebrew/homebrew#28173.

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

This hack is starting to outlive its usefulness, and produces false
positives.

Fixes Homebrew/homebrew#28147.

MacOS.version#<=> is called many, many times during formula loading with
the same half dozen or so arguments. A typical call to this method
involves:

 * a hash lookup to convert a symbol argument to a string
 * creation of a throw-away Version object wrapping the argument
 * the actual version comparison, which is not cheap

This makes it a prime candidate to be memoized.

CLT.installed? previously returned false if the user had outdated dev
tools installed after a 10.9 upgrade, which was confusing to users.

Closes Homebrew/homebrew#27389.
Fixes Homebrew/homebrew#27390.

Closes Homebrew/homebrew#26943.