build: assume a parallel tree layout for unified builds

[compnerd]

Attempt to repair the build for the unified swift build. This allows us
to build a single unified toolchain with swift support. In this layout
assume that cmark and clang are peers of LLVM rather than located in
tools. Doing so allows a uniform layout of the tree and a simpler
build approach.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @gottesmm @Rostepher @shahmishal

[compnerd]

@swift-ci please test

[compnerd]

@lanza you may be interested in this too

[jrose-apple]

I think @llvm-beanz was still interested in having swift/ inside llvm/tools/ (or llvm/projects/ ?) but I'm not sure what that ever did with CMark. Or I could be misremembering.

[compnerd]

@jrose-apple ... so, this actually is somewhat related. This allows me to do a unified build, the difference is that the tree lives as a peer rather than inside of tools. This allows building with -DLLVM_ENABLE_PROJECTS=cmark

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 16a4af0

[llvm-beanz]

When I build with Swift in llvm/tools, I also put CMark there. You can also have Swift and CMark as peers of LLVM in the directory structure and use -DLLVM_EXTERNAL_PROJECTS=swift;cmark -DLLVM_ENABLE_PROJECTS=clang;swift;cmark which leverages the LLVM mono-repo build system tooling.

[compnerd]

@llvm-beanz - hmm, would this change then break that? IIRC, the *_EXTERNAL_SOURCE_DIR are supposed to be setup

[llvm-beanz]

This change will break the workflow of nesting under llvm/tools because the LLVM_EXTERNAL_*_SOURCE_DIR variables aren't set in that case. Personally I don't care if we break that flow because the new mono-repo will make working with LLVM projects nested a thing of the past. As long as the LLVM_EXTERNAL_PROJECTS and LLVM_ENABLE_PROJECTS options work that is good enough.

[compnerd]

@swift-ci please test Linux platform

[davezarzycki]

Did I miss a warning email about this change? I've been using unified builds for a long time, and the error message if one tries to do it the old way (with clang/swift in the tools directory) isn't helpful.

Where are the directions for how to do it the new way?

[EDIT x 2] – @compnerd is helping via the forums. I really wish that this was announced first. A heads up on the forums would have been nice.