[cmake] Clean up add_llvm_subdirectory

I found the pattern of setting the project_BUILD variable to OFF after
processing the project to be pretty confusing. Using global properties
to explicitly keep track of whether a project has been processed or not
seems much more straightforward, and it also allows us to convert the
macro into a function (which is required for the early return).

Factor the project+type+name combination out into a variable while I'm
here, since it's used a whole bunch of times.

I don't believe this should result in any functional changes.

Differential Revision: https://reviews.llvm.org/D55104

llvm-svn: 348180