[MacPorts] #63118: `llvm-12` build failure in macOS Big Sur 11.4

[MacPorts]

#63118: `llvm-12` build failure in macOS Big Sur 11.4
--------------------------+----------------------
  Reporter:  zhiyuanzhai  |      Owner:  kencu@…
      Type:  defect       |     Status:  assigned
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:  2.7.1
Resolution:               |   Keywords:  bigsur
      Port:  llvm-12      |
--------------------------+----------------------

Comment (by kencu):

 I believe you are caught between a commit that broke the llvm-12 build and
 another commit that fixed the build.

 Please try updating your ports tree again -- {{{sudo port -v selfupdate}}}
 -- and then try once again to install llvm-12.

 To give yourself the best chance of success, do this:
 {{{
 sudo port -v clean llvm-12
 sudo port selfupdate
 sudo port -v install llvm-12
 }}}

 and you should find happiness.

-- 
Ticket URL: <https://trac.macports.org/ticket/63118#comment:2>
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