Generate version min load commands when the platform is unknown.

In the case where we are emitting to an object file, the platform is
possibly unknown, and the source object files contained load commands
for version min, we can take the maximum of those min versions and
emit in in the output object file.

This test also tests r259739.

llvm-svn: 259742
diff --git a/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h b/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h
index d2b4806..6dab8ba 100644
--- a/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h
+++ b/lld/lib/ReaderWriter/MachO/MachONormalizedFile.h
@@ -253,6 +253,7 @@
   Hex64                       sourceVersion = 0;
   PackedVersion               minOSverson = 0;
   PackedVersion               sdkVersion = 0;
+  LoadCommandType             minOSVersionKind = (LoadCommandType)0;
 
   // Maps to load commands with LINKEDIT content (final linked images only).
   Hex32                       pageSize = 0;