[CMake] Use LLVM_ENABLE_IDE instead of CMAKE_CONFIGURATION_TYPES

There are several places where we use CMAKE_CONFIGURATION_TYPES to determine if we are using an IDE generator and in turn decide not to generate some of the convenience targets (like all the install-* and check-llvm-* targets). This decision is made because IDEs don't always deal well with the thousands of targets LLVM can generate.

This approach does not work for Visual Studio 15's new CMake integration. Because VS15 uses a Ninja generator, it isn't a multi-configuration build, and generating all these extra targets mucks up the UI and adds little value.

With this change we still don't generate these targets by default for Visual Studio and Xcode generators, and LLVM_ENABLE_IDE becomes a switch that can be enabled on the VS15 CMake builds, to improve the IDE experience.

This is a re-land of r340435, with a few minor fix-ups. The issues causing the revert were addressed in r344218, r344219, and r344553.

llvm-svn: 344555