llvm: update url for HEAD resource libunwind (#3095)

The old URL pointed to the GNU libunwind repository. Unfortunately,
placing the GNU version in the "projects" folder results in CMake being
unable to find the libunwind headers when building with libc++abi,
resulting in a CMake error. The build appears to work fine with the
LLVM version of the library.

Formula/llvm.rb

@@ -108,7 +108,7 @@ class Llvm < Formula
     end
 
     resource "libunwind" do
-      url "git://git.sv.gnu.org/libunwind.git"
+      url "http://llvm.org/git/libunwind.git"
     end
 
     resource "lld" do