Utility: ignore OS version on non-Darwin targets in `ArchSpec`

[compnerd]

Summary

The OS version field is generally not very helpful for non-Darwin
targets. On Linux, it identifies the kernel version which moves
out-of-sync with the userspace. On Windows, this field actually ends up
corresponding to the Visual Studio toolset version instead of the OS
version. Consider non-Darwin targets without an OS version to be fully
specified.

[JDevlieghere]

This is the same change as we did on Github, right? If so this LGTM.

[compnerd]

Correct, this is just upstreaming the original ArchSpec change that we found on Swift.

[kastiglione]

sgtm

lldb/source/Utility/ArchSpec.cpp

bool ArchSpec::IsFullySpecifiedTriple() const {
 
   bool user_triple_fully_specified = false;
 
   if ((user_specified_triple.getOS() != llvm::Triple::UnknownOS) ||
       TripleOSWasSpecified()) {
     if ((user_specified_triple.getVendor() != llvm::Triple::UnknownVendor) ||
         TripleVendorWasSpecified()) {
       const unsigned unspecified = 0;
-      if (user_specified_triple.getOSMajorVersion() != unspecified) {
+      if (!user_specified_triple.isOSDarwin() ||
+          user_specified_triple.getOSMajorVersion() != unspecified) {
         user_triple_fully_specified = true;
       }
     }
   }
 
   return user_triple_fully_specified;
 }