[CMake] Tidy up the organisation of compiler-rt when configured as a standalone

build with an IDE (e.g. Xcode) as the generator.

Previously the global `USE_FOLDERS` property wasn't set in standalone
builds leading to existing settings of FOLDER not being respected.

In addition to this there were several targets that appeared at the top
level that were not interesting and clustered up the view. These have
been changed to be displayed in "Compiler-RT Misc".

Now when an Xcode project is generated from a standalone compiler-rt
build the project navigator is much less cluttered. The interesting
libraries should appear in "Compiler-RT Libraries" in the IDE.

Differential Revision: https://reviews.llvm.org/D48378

llvm-svn: 335728