Stop enabling clang-tools-extra automatically when clang is in LLVM_ENABLE_PROJECTS

If you want to build clang-tools-extra with monorepo, just add it to
LLVM_ENABLE_PROJECTS like with other projects.

See also "Separating clang-tools-extra from clang in LLVM_ENABLE_PROJECTS"
on cfe-dev.

Differential Revision: https://reviews.llvm.org/D58157

llvm-svn: 354057