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.

llvm-svn: 353354