Previously if the user configured their build but then changed

LLVM_ENABLED_PROJECT and reconfigured it had no effect on what
projects were actually built. This was very confusing behaviour. The
reason for this is that the value of the `LLVM_TOOL_<PROJECT>_BUILD`
variables are already set.

The problem here is that we have two sources of truth:

* The projects listed in LLVM_ENABLE_PROJECTS.
* The projects enabled/disabled with LLVM_TOOL_<PROJECT>_BUILD.

At configure time we have no real way of knowing which source of truth
the user wants so we apply the following heuristic:

If the user ever sets `LLVM_ENABLE_PROJECTS` in the CMakeCache then that
is used as the single source of truth and we force the
`LLVM_TOOL_<PROJECT>_BUILD` CMake cache variables to have the
appropriate values that match the contents of the
`LLVM_ENABLE_PROJECTS`. If the user never sets `LLVM_ENABLE_PROJECTS`
then they can continue to use and set the `LLVM_TOOL_<PROJECT>_BUILD`
variables as the "source of truth".

The problem with this approach is that if the user ever tries to use
both `LLVM_ENABLE_PROJECTS` and `LLVM_TOOL_<PROJECT>_BUILD` for the same
build directory then any user set value for `LLVM_TOOL_<PROJECT>_BUILD`
variables will get overwriten, likely without the user noticing.

Hopefully the above shouldn't matter in practice because the
LLVM_TOOL_<PROJECT>_BUILD variables are not documented, but
LLVM_ENABLE_PROJECTS is.

We should probably deprecate the `LLVM_TOOL_<PROJECT>_BUILD`
variables at some point by turning them into to regular CMake
variables that don't live in the CMake cache.

Differential Revision: https://reviews.llvm.org/D57535

llvm-svn: 353148