[flang] Fix for 541f5c4 (D140524) breaking out-of-tree flang build.

Author: hmk46

flang/CMakeLists.txt

@@ -62,6 +62,7 @@ endif()
 
 if (FLANG_STANDALONE_BUILD)
   set(FLANG_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
+  set(CMAKE_INCLUDE_CURRENT_DIR ON)
   if (NOT MSVC_IDE)
     set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
       CACHE BOOL "Enable assertions")