llvm: add compiler-rt to clang build

Closes Homebrew/homebrew#36013.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/llvm.rb

@@ -24,6 +24,11 @@ class Llvm < Formula
       url "http://llvm.org/releases/3.5.1/clang-tools-extra-3.5.1.src.tar.xz"
       sha1 "7a0dd880d7d8fe48bdf0f841eca318337d27a345"
     end
+
+    resource "compiler-rt" do
+      url "http://llvm.org/releases/3.5.1/compiler-rt-3.5.1.src.tar.xz"
+      sha1 "620d59dcc375b24c5663f2793b2bcd74f848435d"
+    end
   end
 
   bottle do
@@ -50,6 +55,10 @@ class Llvm < Formula
     resource "clang-tools-extra" do
       url "http://llvm.org/git/clang-tools-extra.git"
     end
+
+    resource "compiler-rt" do
+      url "http://llvm.org/git/compiler-rt.git"
+    end
   end
 
   # Use absolute paths for shared library IDs
@@ -88,6 +97,7 @@ class Llvm < Formula
       (buildpath/"projects/libcxx").install resource("libcxx")
       (buildpath/"tools/clang").install resource("clang")
       (buildpath/"tools/clang/tools/extra").install resource("clang-tools-extra")
+      (buildpath/"projects/compiler-rt").install resource("compiler-rt")
     end
 
     (buildpath/"tools/lld").install resource("lld") if build.with? "lld"