[flang][runtime] Added Clang CMake modules include path.

Fortran runtime standalone build is broken after D140998
because of the missing CMake modules path. This is a fix.

Differential Revision: https://reviews.llvm.org/D149090

flang/runtime/CMakeLists.txt

@@ -26,6 +26,7 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
 
   set(LLVM_COMMON_CMAKE_UTILS "${FLANG_SOURCE_DIR}/../cmake")
   set(LLVM_CMAKE_UTILS "${FLANG_SOURCE_DIR}/../llvm/cmake")
+  set(CLANG_CMAKE_UTILS "${FLANG_SOURCE_DIR}/../clang/cmake")
 
   # Add path for custom modules
   list(INSERT CMAKE_MODULE_PATH 0
@@ -35,8 +36,10 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
     "${LLVM_COMMON_CMAKE_UTILS}/Modules"
     "${LLVM_CMAKE_UTILS}"
     "${LLVM_CMAKE_UTILS}/modules"
+    "${CLANG_CMAKE_UTILS}/modules"
     )
 
+  include(AddClang)
   include(AddLLVM)
   include(AddFlang)
   include(HandleLLVMOptions)