Bump useable clang version to 318

Build 211 can build most things, but I've seen intermittent
miscompilation and slower code. Also it comes from Xcode 4.2, so most
people don't have it anyway. Let's use 218 as the minimum viable
version.

Library/Homebrew/compilers.rb

@@ -67,7 +67,7 @@ class CompilerSelector
 
   def priority_for(cc)
     case cc
-    when :clang then MacOS.clang_build_version >= 211 ? 3 : 0.5
+    when :clang then MacOS.clang_build_version >= 318 ? 3 : 0.5
     when :llvm  then 2
     when :gcc   then 1
     end

Library/Homebrew/test/test_compilers.rb

@@ -10,7 +10,7 @@ class CompilerTests < Test::Unit::TestCase
     assert f.fails_with? :llvm
     assert !(f.fails_with? :gcc)
     assert_equal case MacOS.clang_build_version
-      when 0..210 then :gcc
+      when 0..318 then :gcc
       else :clang
       end, cs.compiler
   end