Revise IDE folder structure (#89755)

Update the folder titles for targets in the monorepository that have not
seen taken care of for some time. These are the folders that targets are
organized in Visual Studio and XCode
(`set_property(TARGET <target> PROPERTY FOLDER "<title>")`)
when using the respective CMake's IDE generator.

 * Ensure that every target is in a folder
 * Use a folder hierarchy with each LLVM subproject as a top-level folder
 * Use consistent folder names between subprojects
 * When using target-creating functions from AddLLVM.cmake, automatically
deduce the folder. This reduces the number of
`set_property`/`set_target_property`, but are still necessary when
`add_custom_target`, `add_executable`, `add_library`, etc. are used. A
LLVM_SUBPROJECT_TITLE definition is used for that in each subproject's
root CMakeLists.txt.

NOKEYCHECK=True
GitOrigin-RevId: e14f5f225aedbb829996e33a6e3d0a4dd3d06e0d
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f34cb17..cb5e0e5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,6 +5,7 @@
 # Setup Project
 #===============================================================================
 cmake_minimum_required(VERSION 3.20.0)
+set(LLVM_SUBPROJECT_TITLE "libc++")
 
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")