A little cleanup in ObjectFileMachO::GetSDKVersion.

This method does one of two things:

1. finds a minimum os deployment version # in a Mach-O load
command and saves the three parts in the m_sdk_version, or

2. finds no valid min os version # load command, pushes a 
sentinel value on the m_sdk_version vector so we don't search
the same load commands multiple times.

There was a little bug when we found a load command with 
a version of 0.0.0 - the method would not add anything to
the m_sdk_version vector but would declare that a success.
It would not push the sentinel value to the vector.
There was code later in the method which assumed that
the vector always had a sentinel value, at least, and that
code could crash when this method was called back when
evaluating a Swift expression.  (these version #'s are 
fetched lazily so it wouldn't happen when the object file
was parsed, only when doing an expression that needed
the version #).

<rdar://problem/41372699> 

llvm-svn: 335556