ENH: Update Xcode Apple Clang compiler for nightly dashboard

Suggested by Sean McBride, as he wrote about Xcode 7.2.1:
> It's the newest possible Xcode on macOS 10.10.x, which is the oldest
> version of macOS that I have a bot for.

Details taken from
https://trac.macports.org/wiki/XcodeVersionInfo#Xcode7.2.11

SoftwareGuide/Latex/Introduction/Installation.tex

@@ -35,8 +35,7 @@ \chapter{Configuring and Building ITK}
 \begin{itemize}
 \item GCC 5.1 and newer
 \item Visual Studio 2017 and 2019, toolsets v141 and v142
-\item Apple Clang 7.0 and newer
-\item Clang 6 and newer
+\item Xcode 7.2.1 Apple LLVM Clang version 7.0.2 (clang-700.1.81) and newer
 \end{itemize}
 
 If you are currently using an outdated compiler this may be an excellent excuse