[libc++abi] Improve error message when libunwind is missing from LLVM_ENABLE_RUNTIMES (#77991)

This doesn't actually prevent the build from failing, but it provides a
better diagnostic that explains what is needed in order to fix the build.

Fixes #77846
Fixes #77843

Author: ldionne

libcxxabi/CMakeLists.txt

@@ -47,6 +47,9 @@ option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode.
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." OFF)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." ON)
+if (LIBCXXABI_USE_LLVM_UNWINDER AND NOT "libunwind" IN_LIST LLVM_ENABLE_RUNTIMES)
+  message(FATAL_ERROR "LIBCXXABI_USE_LLVM_UNWINDER is set to ON, but libuwnind is not specified in LLVM_ENABLE_RUNTIMES.")
+endif()
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 option(LIBCXXABI_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)