brew doctor: add check for outdated compilers

A common source of build problems on Xcode 4.3+ is outdated compilers,
usually when a user has installed over top of an old version and hasn't
installed the CLT. Since the compilers from the previous Xcode are still
around, brew doctor wouldn't complain.

This adds a hash containing a list of the canonical compiler versions
for supported versions of Xcode, and adds a check against that to determine
whether a given installation has any compilers which are out of date.

Closes #11518.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

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