[cmake] Convert the NATIVE llvm build process to be project agnostic

[cmake] Convert the NATIVE llvm build process to be project agnostic

lldb recently added a tablegen tool. In order to properly cross compile
lldb standalone there needs to be a mechanism to generate the native
lldb build, analgous to what's done for the NATIVE llvm build. Thus,
we can simply modify this setup to allow for any project to be used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366514 91177308-0d34-0410-b5e6-96231b3b80d8