Fix CMake error whet llvm-config reports a non-existent source directory.

llvm-svn: 244717
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 62d526bd81472fe46609dbfdb1f968361b94e582
diff --git a/cmake/Modules/HandleOutOfTreeLLVM.cmake b/cmake/Modules/HandleOutOfTreeLLVM.cmake
index bf629a9..53adb06 100644
--- a/cmake/Modules/HandleOutOfTreeLLVM.cmake
+++ b/cmake/Modules/HandleOutOfTreeLLVM.cmake
@@ -42,11 +42,15 @@
   endif()
 
   if (NOT EXISTS ${LLVM_MAIN_SRC_DIR})
-    message(FATAL_ERROR "Not found: ${LLVM_MAIN_SRC_DIR}")
+    set(LLVM_FOUND OFF)
+    message(WARNING "Not found: ${LLVM_MAIN_SRC_DIR}")
+    return()
   endif()
 
   if(NOT EXISTS ${LLVM_CMAKE_PATH})
-    message(FATAL_ERROR "Not found: ${LLVM_CMAKE_PATH}")
+    set(LLVM_FOUND OFF)
+    message(WARNING "Not found: ${LLVM_CMAKE_PATH}")
+    return()
   endif()
 
   list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")