[CMake] We shouldn't be storing values in the cache unless they actually need CMake...

[CMake] We shouldn't be storing values in the cache unless they actually need CMake cache behavior.

add_llvm_external_project puts LLVM_EXTERNAL_${nameUPPER}_SOURCE_DIR into the cache even if it is just the in-tree default path. This causes all sorts of oddness, and makes it so that I can't change the behavior of this variable.

This patch never puts LLVM_EXTERNAL_${nameUPPER}_SOURCE_DIR into the cache. It will only end up in the cache if it is specified on the command line, which is the correct behavior.

There is also a temporary change to remove non-default values from the cache if they are already present. This should have the impact of cleaning out unncecissary values from the caches on the buildbots and people's local build directories. This part of the change is marked with a TODO and can be removed in a few days.

llvm-svn: 242102