Updating macOS Version logic for macOS 11+ and Darwin 20+

[AndrewLitteken]

Summary

Minor releases of macOS 11+ increment the minor field rather than the micro field at the same rate as the Darwin minor field. Currently the getMacOSHostVersion test fails for macOS 11.1.

This adjusts the logic for the found macOS minor version from the Darwin version to be one less than the Darwin minor version unless the Darwin minor version, is zero, in which case the macOS minor version is also zero. This is a guess based on the past few releases: https://en.wikipedia.org/wiki/Darwin_(operating_system).

[AndrewLitteken]

Closing since has been solved differently

llvm/lib/Support/Triple.cpp

     if (Major < 4)
       return false;
     if (Major <= 19) {
       Micro = 0;
       Minor = Major - 4;
       Major = 10;
     } else {
       Micro = 0;
-      Minor = 0;
+      // macOS 11+ minor version trails darwin20+ version by 1, unless 0.
+      Minor = Minor == 0 ? 0 : Minor - 1;
       // darwin20+ corresponds to macOS 11+.
       Major = 11 + Major - 20;
     }
     break;
   case MacOSX:
     // Default to 10.4.
     if (Major == 0) {
       Major = 10;