[CMake] Change the default value of LLVM_ENABLE_IDE

There really aren't any generator behaviors that we need to take `CMAKE_EXTRA_GENERATOR` into account for. Where we need to take different behaviors for IDEs is mostly in enabling or disabling certain build system features that are optional but trip up the IDE UIs. Like the generation of lots of utility targets.

By changing the LLVM_ENABLE_IDE default to only being on for multi-configuration generators, we allow gating where it will impact the UI presentation, while also supporting optionally disabling the generation if your tooling workflow encounters problems. Presently being able to manually disable extra target generation is useful for Visual Studio 2017's CMake integration where the IDE has trouble displaying and working with the large number of optional targets.

llvm-svn: 344553