Provide a meaningful diagnostic when LLVM_PATH doesn't point to a directory

git-svn-id: http://llvm.org/svn/llvm-project/libcxxabi/trunk@369312 91177308-0d34-0410-b5e6-96231b3b80d8

Author: ldionne

cmake/Modules/HandleOutOfTreeLLVM.cmake

@@ -9,6 +9,9 @@ macro(find_llvm_parts)
     set(LLVM_PATH ${LLVM_PATH} CACHE PATH "Path to LLVM source tree")
     set(LLVM_MAIN_SRC_DIR ${LLVM_PATH})
     set(LLVM_CMAKE_PATH "${LLVM_PATH}/cmake/modules")
+    if (NOT IS_DIRECTORY "${LLVM_PATH}")
+      message(FATAL_ERROR "The provided LLVM_PATH (${LLVM_PATH}) is not a valid directory")
+    endif()
   elseif(LLVM_CONFIG_PATH)
     message(STATUS "Found LLVM_CONFIG_PATH as ${LLVM_CONFIG_PATH}")
     set(LIBCXXABI_USING_INSTALLED_LLVM 1)