[Mono] Upgrade LLVM to 11.0

[SamMonoRT]

Tracking issue to upgrade LLVM from 9.x to 11.0 to consume newer features

• Forward-port mono-specific patches to LLVM 11
• Ensure dotnet/runtime builds against a local copy of patched LLVM 11
• Enable running dotnet/runtime against a custom build of LLVM 11 on CI
• Switch to using LLVM 11 in production

Not strictly needed

• Verify that Apple bitcode submission works with bitcode generated by patched LLVM 11

[SamMonoRT]

Local branch for upgrade to LLVM 11: https://github.com/imhameed/llvm-project/tree/ih-llvm11-test

[imhameed]

I cherry-picked all the mono-specific patches we made to LLVM 9 and applied them to LLVM 11. This can be seen at https://github.com/imhameed/llvm-project/tree/ih-llvm11-test. The result doesn't build yet, and I am debugging that now.

[imhameed]

Now LLVM 11 fails to build with:

[...]
Scanning dependencies of target benchmark_main
make[2]: *** No rule to make target `utils/llvm-locstats/llvm-dwarfdump', needed by `bin/llvm-locstats'.  Stop.
make[1]: *** [utils/llvm-locstats/CMakeFiles/llvm-locstats.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

EDIT: this is resolved.

[imhameed]

The work-in-progress runtime-side changes can be found here: https://github.com/imhameed/runtime/tree/mono-llvm11

[imhameed]

dotnet/runtime now builds against LLVM 11 for me locally. My changes are here: #51993

[imhameed]

The CI test suite is being run against LLVM 11 here: #52984

There are some genuine failures on the LLVM AOT lanes for arm64 and amd64. I don't have access to my usual Linux work setup right now, and I can't reproduce these failures on amd64 Catalina, so I'm setting up a little Linux VM to debug this further.

[SamMonoRT]

@imhameed - please verify that all required updates are reflected in main and we can close this issue.

[imhameed]

This should be done.