[MacPorts] #70143: MacPorts can choose the wrong compiler if Xcode and Xcode command line tools versions differ

[MacPorts]

#70143: MacPorts can choose the wrong compiler if Xcode and Xcode command line
tools versions differ
------------------------+-----------------------------
 Reporter:  ryandesign  |      Owner:  (none)
     Type:  defect      |     Status:  new
 Priority:  Normal      |  Milestone:  MacPorts Future
Component:  base        |    Version:  2.9.3
 Keywords:              |       Port:
------------------------+-----------------------------
 MacPorts knows that C++20 requires Apple clang 1500 or later:

 https://github.com/macports/macports-
 base/blob/9c81555a5e2ddce4a03488918cba3661be262904/src/port1.0/portconfigure.tcl#L949-L950

 This is available in Xcode 15 or later or the Xcode command line tools 15
 or later.

 The macOS 13 GitHub Actions CI runner defaults to having Xcode 15.0.1 and
 the Xcode command line tools 14.3 installed (see #69668) thus its Xcode
 clang has C++20 support while its Xcode command line tools clang does not
 have C++20 support. But MacPorts evidently decides that it's ok to use the
 Xcode command line tools clang for a C++20-requiring port on the basis of
 the Xcode clang version. This caused the build failure of a port that
 requires C++20 (nghttp2-tools) on the macOS 13 runner in
 https://github.com/macports/macports-ports/pull/24273.

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Ticket URL: <https://trac.macports.org/ticket/70143>
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