[CMake] Polish folders in generated Xcode project

Summary: Group plugins by subfolder. Move liblldb-resource-headers to `lldb misc`. Avoid install-distribution related targets in IDE-enabled builds.

Reviewers: jingham, mib, stella.stamenova

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64865

llvm-svn: 366459