[libc++] Set CMAKE_FOLDER. NFC.

* This variable populates the default value of FOLDER target property. It is used in some IDE's (e.g. MSVC) to group different targets together.

libcxx/CMakeLists.txt

@@ -19,6 +19,8 @@ set(CMAKE_MODULE_PATH
   ${CMAKE_MODULE_PATH}
   )
 
+set(CMAKE_FOLDER "libc++")
+
 set(LIBCXX_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBCXX_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
 set(LIBCXX_BINARY_INCLUDE_DIR "${LIBCXX_BINARY_DIR}/include/c++build")

libcxx/benchmarks/CMakeLists.txt

@@ -5,6 +5,8 @@ include(CheckCXXCompilerFlag)
 # Build Google Benchmark for libc++
 #==============================================================================
 
+set(CMAKE_FOLDER "${CMAKE_FOLDER}/Benchmarks")
+
 set(BENCHMARK_LIBCXX_COMPILE_FLAGS
     -Wno-unused-command-line-argument
     -nostdinc++

libcxxabi/CMakeLists.txt

@@ -17,6 +17,8 @@ set(CMAKE_MODULE_PATH
   ${CMAKE_MODULE_PATH}
   )
 
+set(CMAKE_FOLDER "libc++")
+
 set(LIBCXXABI_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBCXXABI_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
 set(LIBCXXABI_LIBCXX_PATH "${CMAKE_CURRENT_LIST_DIR}/../libcxx" CACHE PATH