Offer alternative for cxxabi on Windows (Swift+Clang).

[furby-tm]

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Summarize your change.

This fixes the clang compilation of OpenTimelineIO on Microsoft Windows, when compiling with Swift. Previously, the cxxabi.h header was erroneously getting included in the Windows clang compilation of OpenTimelineIO's stringUtils.cpp file because it was being conditionally compiled for clang across all platforms, and this header does not exist on the Windows platform.

• The include for cxxabi.h is now guarded via __has_include(<cxxabi.h>).
• A new preprocessor definition for OTIO_HAVE_DEMANGLER is set to 1 if cxxabi.h exists, and 0 if this header does not exist.
• Previous checks against defined(__GNUC__) || defined(__clang__) are replaced with conditional compilation checks against the new OTIO_HAVE_DEMANGLER preprocessor define.

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: furby-tm / name: furby™ (69144f24f6dd1f66b3b75047f4d2ea3fbdf20daa)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.71%. Comparing base (c0e97b0) to head (69144f2). Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1787      +/-   ##
==========================================
- Coverage   84.11%   81.71%   -2.41%     
==========================================
  Files         198      176      -22     
  Lines       22241    12319    -9922     
  Branches     4687     3022    -1665     
==========================================
- Hits        18709    10066    -8643     
+ Misses       2610     1717     -893     
+ Partials      922      536     -386     

Flag	Coverage Δ
py-unittests	81.71% <ø> (-2.41%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/opentimelineio/stringUtils.cpp	44.44% <ø> (ø)

... and 67 files with indirect coverage changes

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[furby-tm]

If the team prefers to remove the cxxabi demangling, I can update this PR to deprecate it instead. Please feel free to ping me in this issue once the team has reached a consensus.

[jminor]

From some offline discussion, it seems there is no objection to removing the demangling. @furby-tm we would gladly accept this PR as-is, or with the demangling removed. Please let us know which you'd like.

[furby-tm]

I'll go ahead and remove the demangling @jminor

[furby-tm]

Closing in favor of https://github.com/AcademySoftwareFoundation/OpenTimelineIO/pull/1800