Fix clang URL in x-compilation script, only available via HTTPS now (#2834)

Adding `-L` to curl also works ...

Without this, the toolchain build silently fails.

Author: helje5

Utilities/build_ubuntu_cross_compilation_toolchain

@@ -103,7 +103,7 @@ blocks_h_url="https://raw.githubusercontent.com/apple/swift-corelibs-libdispatch
 xc_tc_name="swift.xctoolchain"
 linux_sdk_name="ubuntu-xenial.sdk"
 cross_tc_basename="cross-toolchain"
-clang_package_url="http://releases.llvm.org/6.0.0/clang+llvm-6.0.0-x86_64-apple-darwin.tar.xz"
+clang_package_url="https://releases.llvm.org/6.0.0/clang+llvm-6.0.0-x86_64-apple-darwin.tar.xz"
 ubuntu_mirror="http://gb.archive.ubuntu.com/ubuntu"
 packages_file="$ubuntu_mirror/dists/xenial/main/binary-amd64/Packages.gz"
 pkg_names=( libc6-dev linux-libc-dev libicu55 libgcc-5-dev libicu-dev libc6 libgcc1 libstdc++-5-dev libstdc++6 zlib1g-dev )