[MacPorts] #62927: Prefer MacPorts clang-11 and clang-10 over clang-9.0

[MacPorts]

#62927: Prefer MacPorts clang-11 and clang-10 over clang-9.0
-------------------------+--------------------
 Reporter:  ryandesign   |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  base         |    Version:  2.6.4
 Keywords:               |       Port:
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 If Xcode clang is blacklisted, base picks MacPorts clang-9.0 as its first
 fallback. Newer stable versions should be preferred, but see comments from
 Ken in [2f37547eddee14389286364929d2b2f2b57b321c/macports-ports] about
 some complications.

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Ticket URL: <https://trac.macports.org/ticket/62927>
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