Update scripts/Xcode/build-llvm.py to use llvm/clang 4 branch

git-svn-id: https://llvm.org/svn/llvm-project/lldb/branches/release_40@291842 91177308-0d34-0410-b5e6-96231b3b80d8

scripts/Xcode/build-llvm.py

diff --git a/scripts/Xcode/build-llvm.py b/scripts/Xcode/build-llvm.py
index 792f925..c00907e 100755
--- a/scripts/Xcode/build-llvm.py
+++ b/scripts/Xcode/build-llvm.py
@@ -22,12 +22,12 @@
 
 
 def LLVM_REF():
-    llvm_ref = "master"
+    llvm_ref = "release_40"
     return llvm_ref
 
 
 def CLANG_REF():
-    clang_ref = "master"
+    clang_ref = "release_40"
     return clang_ref
 
 # For use with Xcode-style builds