[cmake] Remove llvm from LLVM_ALL_PROJECTS

LLVM is always built; including it in LLVM_ENABLE_PROJECTS has no
effect, but since it's in LLVM_ALL_PROJECTS, we produce a confusing
message about it being disabled. Drop it from LLVM_ALL_PROJECTS to avoid
this. Pointed out by David Greene on the mailing list [1].

[1] http://lists.llvm.org/pipermail/llvm-dev/2019-March/130854.html

llvm-svn: 355735