With the new LLVM_ENABLE_RUNTIMES option introduced in https://reviews.llvm.org/D40233, compiler-rt can now be included as a runtime. Since compiler-rt is needed for PGO, runtimes needs to be included as a dependency of clang-bootstrap-deps when building the stage1 compiler.
I don't think we can add this dependency unconditionally. If LLVM_ENABLE_RUNTIMES isn't set, runtimes list would end up empty here https://github.com/llvm/llvm-project/blob/master/llvm/runtimes/CMakeLists.txt#L32, in which case we wouldn't define runtimes target at all here https://github.com/llvm/llvm-project/blob/master/llvm/runtimes/CMakeLists.txt#L497.
We also cannot just check whether LLVM_ENABLE_RUNTIMES is set because in theory we still support the nested layout where users can place runtimes directly into llvm/runtimes subdirectory; we probably don't need this now that we're in monorepo, but we shouldn't break it until we officially announce the deprecation of that feature.
We're not adding this dependency unconditionally. This line is under an if(runtimes) block, so at this point where we're adding runtimes to clang-bootstrap-deps, the runtimes list won't be empty and the runtimes target would be defined. Additionally, if the runtimes target wasn't defined, then the runtimes-configure target which is referenced in the line above also wouldn't be defined, but since that line works fine, adding an additional dependency on runtimes should be fine.