bootstrap: always build llvm's dylib when linking to it

[xSetech]

The bootstrap example says that LLVM's dynamic library will be built when llvm.link-shared is true. It actually only set the flag that enables linking against the library (LLVM_LINK_LLVM_DYLIB). It should also set LLVM_BUILD_LLVM_DYLIB.

On clean builds, LLVM's configuration helpfully sets the BUILD flag to true if the LINK flag is true. If a dev toggles link-shared from false to true after an initial build, LINK will be "ON" (given by bootstrap) and BUILD will be "OFF" (from the cmake cache). The library to link will not be built, causing the build to fail at link time.

With this change, the library is always built when it's to be linked. Conversely, it is not built if it is not to be linked.

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[rustbot]

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

[rustbot]

r? @clubby789

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[xSetech]

@nikic I moved LLVM_VERSIONED_DYLIB_NAME_ON_DARWIN=OFF out of the block so it's always off. Seems like that was your intent from #158734, but let me know if it actually depends on llvm_link_shared()

[Kobzol]

On clean builds, LLVM's configuration helpfully sets the BUILD flag to true if the LINK flag is true. If a dev toggles link-shared from false to true after an initial build, LINK will be "ON" (given by bootstrap) and BUILD will be "OFF" (from the cmake cache). The library to link will not be built, causing the build to fail at link time.

I wonder how does that show up? Because if LLVM was already built locally, and you modify the LLVM bootstrap settings, CMake won't be involved at all, because bootstrap currently caches the whole build and doesn't examine whether the settings changed or not.