[cmake] Drop clang-tools-extra from LLVM_ALL_PROJECTS

[cmake] Drop clang-tools-extra from LLVM_ALL_PROJECTS

We iterate over the list and only enable projects from that list that
are present in LLVM_ENABLE_PROJECTS and disable all other projects. Most
users will only specify clang in LLVM_ENABLE_PROJECTS and expect
clang-tools-extra to be implicitly enabled, so remove clang-tools-extra
from LLVM_ALL_PROJECTS so that it doesn't get disabled instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353354 91177308-0d34-0410-b5e6-96231b3b80d8