[projects] Use add_llvm_external_project for implicit projects

This allows disabling implicit projects via the LLVM_TOOL_*_BUILD
variables, similar to how implicit tools can be disabled. They'll still
be enabled by default, since add_llvm_external_project defaults the
LLVM_TOOL_*_BUILD variables to ON for in-tree implciit projects.

Differential Revision: https://reviews.llvm.org/D55105

llvm-svn: 348064