Add comment about the importance of being adjacent. NFC

llvm-svn: 240328

clang/lib/Driver/ToolChains.cpp

@@ -2896,6 +2896,9 @@ Tool *Solaris::buildLinker() const {
 /// Distribution (very bare-bones at the moment).
 
 enum Distro {
+  // NB: Releases of a particular Linux distro should be kept together
+  // in this enum, because some tests are done by integer comparison against
+  // the first and last known member in the family, e.g. IsRedHat().
   ArchLinux,
   DebianLenny,
   DebianSqueeze,