A little more context for people who haven't been through this before:
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Swift depends on LLVM and other repositories, which are themselves open source projects that get lots of updates on a day-to-day basis.
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To avoid churn for Swift developers, we periodically cut release branches for LLVM etc. This does eventually form the version of LLVM used in a Swift release.
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The "stable" branch for LLVM (that Swift's "master" branch builds against) is technically a different branch, but in practice it has the same content as "the latest release branch".
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The operation that Mishal's going to perform is to move the "stable" branch of LLVM and others from swift-5.0-branch to swift-5.1-branch, and update Swift to build against this new content.
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LLDB is extra complicated because it is from the LLVM project but also has parts that depend on Swift.
You can read more about this (from a Swift-repo-centric point of view) in docs/Branches.md.