diagnostic: only make minimum Xcode/CLT version checks fatal.

Also: freeze the relevant arrays.

Library/Homebrew/diagnostic.rb

@@ -86,12 +86,13 @@ module Homebrew
       def development_tools_checks
         %w[
           check_for_installed_developer_tools
-        ]
+        ].freeze
       end
 
       def fatal_development_tools_checks
         %w[
-        ]
+        ].freeze
+      end
       end
 
       def check_for_installed_developer_tools

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -10,18 +10,17 @@ module Homebrew
           check_xcode_license_approved
           check_for_osx_gcc_installer
           check_xcode_8_without_clt_on_el_capitan
-        ]
+          check_xcode_up_to_date
+          check_clt_up_to_date
+          check_for_other_package_managers
+        ].freeze
       end
 
       def fatal_development_tools_checks
-        if MacOS.version >= :sierra && ENV["CI"].nil?
         %w[
-            check_xcode_up_to_date
-            check_clt_up_to_date
-          ]
-        else
-          %w[
-          ]
+          check_xcode_minimum_version
+          check_clt_minimum_version
+        ].freeze
       end
       end