If LLVM isn't installed then fails_with_llvm? is true

This makes sense, I assure you. Fixes Homebrew/homebrew#7458.

Library/Homebrew/formula.rb

@@ -231,7 +231,7 @@ class Formula
   def fails_with_llvm?
     llvm = self.class.fails_with_llvm_reason
     if llvm
-      if llvm.build and MacOS.llvm_build_version > llvm.build.to_i
+      if llvm.build and MacOS.llvm_build_version.to_i > llvm.build.to_i
         false
       else
         llvm

Library/Homebrew/utils.rb

@@ -329,7 +329,7 @@ module MacOS extend self
   def llvm_build_version
     # for Xcode 3 on OS X 10.5 this will not exist
     # NOTE may not be true anymore but we can't test
-    if File.exist? "/usr/bin/llvm-gcc"
+    @llvm_build_version ||= if File.exist? "/usr/bin/llvm-gcc"
       `/usr/bin/llvm-gcc -v 2>&1` =~ /LLVM build (\d{4,})/
       $1.to_i
     end