[cmake] Add all subprojects to LLVM_ALL_PROJECTS

Make LLVM_ALL_PROJECTS reflect all top-level directories in the monorepo
rather than an arbitrary subset. clang-tools-extra is technically
unnecessary since it gets enabled by clang, but having it there for
consistency shouldn't hurt either.

Differential Revision: https://reviews.llvm.org/D57843

llvm-svn: 353346