Extract XcodeVersion to be a standalone module function

Extract XcodeVersion to be a standalone module function

The _XcodeVersion from XcodeSettings class does not need to be a member
function as it does not use any of the field of the class. Extract it
to be a standalone function at the module level, and all the functions
it calls (that also don't use fields from XcodeSettings).

Keep the _XcodeVersion wrapper function that keeps a cache of the
values returned from Xcode to not degrade performance for clients of
the old API that may depends on that cache.

BUG=None
R=mark@chromium.org, thakis@chromium.org

Committed: https://code.google.com/p/gyp/source/detail?r=1849

[sdefresne, 2014-02-11 10:38:40]

In my issue 138533006 (https://codereview.chromium.org/138533006/)
I needed to use XcodeVersion without having an XcodeSettings object and
turned the method to classmethod.

This change instead extract the method to be standalone module function
as it does not use an field member. It keeps the wrapper function so that
the cache is still present (as some of the caller may themselves be called
frequently).

I'll rebase my CL on top of that one if it is approved.

[sdefresne, 2014-02-11 10:39:11]

+cc: justincohen

[Mark Mentovai, 2014-02-11 14:10:44]

LGTM

[sdefresne, 2014-02-11 16:15:16]

Committed patchset #1 manually as r1849 (presubmit successful).