Update to LLVM 11.0.0.rc1

[isuruf]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[isuruf]

@conda-forge-admin, rerender

[chrisburr]

The aarch64 build failed with this error:

[100%] Linking CXX executable ../../bin/clang-tblgen
/home/conda/feedstock_root/build_artifacts/clang_packages_1602660595481/_build_env/bin/../lib/gcc/x86_64-conda-linux-gnu/7.5.0/../../../../x86_64-conda-linux-gnu/bin/ld: skipping incompatible /home/conda/feedstock_root/build_artifacts/clang_packages_1602660595481/_build_env/lib/libz.so when searching for -lz
/home/conda/feedstock_root/build_artifacts/clang_packages_1602660595481/_build_env/bin/../lib/gcc/x86_64-conda-linux-gnu/7.5.0/../../../../x86_64-conda-linux-gnu/bin/ld: skipping incompatible /home/conda/feedstock_root/build_artifacts/clang_packages_1602660595481/_build_env/lib/libz.a when searching for -lz
/home/conda/feedstock_root/build_artifacts/clang_packages_1602660595481/_build_env/bin/../lib/gcc/x86_64-conda-linux-gnu/7.5.0/../../../../x86_64-conda-linux-gnu/bin/ld: cannot find -lz
/home/conda/feedstock_root/build_artifacts/clang_packages_1602660595481/_build_env/bin/../lib/gcc/x86_64-conda-linux-gnu/7.5.0/../../../../x86_64-conda-linux-gnu/bin/ld: skipping incompatible /home/conda/feedstock_root/build_artifacts/clang_packages_1602660595481/_build_env/lib/libz.so when searching for -lz
collect2: error: ld returned 1 exit status

I can't reproduce it locally so I'll try restarting

[chrisburr]

And now it's struggling to download the macOS 11 SDK:

+++ curl -L -O https://github.com/phracker/MacOSX-SDKs/releases/download/10.15/MacOSX11.0.sdk.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100     9  100     9    0     0     89      0 --:--:-- --:--:-- --:--:--    89
++++ dirname /Applications/Xcode_12_beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk
+++ tar -xf MacOSX11.0.sdk.tar.xz -C /Applications/Xcode_12_beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
tar: Error opening archive: Unrecognized archive format

Caused by conda-forge/conda-forge-ci-setup-feedstock#126

Though the aarch64 error has vanished at least