Fix pinned builds error due to obsolete LLVM repo

[bogniq]

Change Description

This PR fixes the pinned builds error for release/2.0.x, which was caused by using the no longer existing LLVM repo https://git.llvm.org/git. The new build scripts use a mirror repo https://github.com/llvm-mirror that maintains the same commit hashes as git.llvm.org.

To keep the binary's compiler version info (including Clang repo's origin URL) consistent with existing release binaries, after the LLVM and Clang repos are checked out, their remote origin URLs are modified to be https://git.llvm.org/git.

Change Type

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• Stability bug fix

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Testing Changes

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Consensus Changes

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API Changes

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Documentation Additions

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[nksanthosh]

Please ensure that this build passes https://buildkite.com/EOSIO/eosio-build-scripts/builds/1210 (build script pipeline) before merging.