[CMake] Make llvm-config implicit dependency for subprojects

The subproject interface being used for runtime libraries expects that llvm-config is passed into the subproject for consumption. We currently do this for every subproject, so we should expect that all LLVM ExternalProjects depend on llvm-config for the time being.

Eventually I'd like to see the sub-projects using LLVMConfig.cmake instead of the llvm-config binary, but that will take time to roll out.

llvm-svn: 279155