[Order Files] Fix order file usage

The CMake EXISTS operator needs to have the variable expanded. Not expanding this was causing the if statement to always be false, which made it impossible to pass linker order files in.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264757 91177308-0d34-0410-b5e6-96231b3b80d8

tools/driver/CMakeLists.txt

@@ -87,7 +87,7 @@ if (APPLE)
   set(TOOL_INFO_BUILD_VERSION)
 endif()
 
-if(CLANG_ORDER_FILE AND EXISTS CLANG_ORDER_FILE)
+if(CLANG_ORDER_FILE AND EXISTS ${CLANG_ORDER_FILE})
   target_link_libraries(clang "-Wl,-order_file,${CLANG_ORDER_FILE}")
 endif()