[MacPorts] #55548: compilers portgroup: remove clang 3.5 and 3.6

[MacPorts]

#55548: compilers portgroup: remove clang 3.5 and 3.6
------------------------+---------------------------------
 Reporter:  ryandesign  |      Owner:  MarcusCalhoun-Lopez
     Type:  defect      |     Status:  new
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:
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 The compilers portgroup allows the creation of clang35 and clang36
 variants, but the clang-3.5 and clang-3.6 ports are no longer installable
 (they are replaced by clang-5.0) so these variants should be removed.

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Ticket URL: <https://trac.macports.org/ticket/55548>
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