[libc++abi] Provide an explicit error when trying to build for MSVC (l…

…lvm#119370)

Fixes llvm#119322

libcxxabi/CMakeLists.txt

@@ -34,6 +34,11 @@ MACRO_ENSURE_OUT_OF_SOURCE_BUILD(
  build directory and run 'cmake /path/to/${PROJECT_NAME} [options]' there."
  )
 
+if (MSVC)
+  message(FATAL_ERROR "Libc++abi can't be built for MSVC targets, and doing so is pointless anyway because such "
+                      "targets must use the MS C++ ABI, and libc++abi provides the Itanium C++ ABI.")
+endif()
+
 #===============================================================================
 # Setup CMake Options
 #===============================================================================

libunwind/CMakeLists.txt

@@ -24,6 +24,11 @@ set(LIBUNWIND_LIBCXX_PATH "${CMAKE_CURRENT_LIST_DIR}/../libcxx" CACHE PATH
 include(GNUInstallDirs)
 include(CheckSymbolExists)
 
+if (MSVC)
+  message(FATAL_ERROR "Libunwind doesn't build for MSVC targets, and that is almost certainly not what you want to do "
+                      "anyway since libunwind is tied to the Itanium C++ ABI, and MSVC targets must use the MS C++ ABI.")
+endif()
+
 #===============================================================================
 # Setup CMake Options
 #===============================================================================