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

llvm-svn: 369312
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 5a14c1722183bbe3224f8c69b8e189326185522a

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)