[flang] Add Clang CMake modules path for Decimal library

[jpenix-quic]

Summary

This fixes an error that clang_target_link_libraries is unknown
when building the Decimal library standalone--this is the same
as D149090, just for the Decimal library (vs the runtime).

I was still seeing this error for the Decimal library and didn't
see a corresponding patch--apologies in advance if I missed one
or if this is a duplicate!

[vzakhari]

Thank you for the fix!

[jpenix-quic]

No problem, thank you for the review and for the original runtime fix!

flang/lib/Decimal/CMakeLists.txt

 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   cmake_minimum_required(VERSION 3.13.4)
 
   project(FortranDecimal C CXX)
 
   set(CMAKE_CXX_STANDARD 17)
   set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
   set(CMAKE_CXX_EXTENSIONS OFF)
 
   set(FLANG_SOURCE_DIR "${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
     "${FLANG_SOURCE_DIR}/cmake"
     "${FLANG_SOURCE_DIR}/cmake/modules"
     "${LLVM_COMMON_CMAKE_UTILS}"
     "${LLVM_COMMON_CMAKE_UTILS}/Modules"
     "${LLVM_CMAKE_UTILS}"
     "${LLVM_CMAKE_UTILS}/modules"
+    "${CLANG_CMAKE_UTILS}/modules"
     )
 
+  include(AddClang)
   include(AddLLVM)
   include(AddFlang)
   include(HandleLLVMOptions)
 
   include(TestBigEndian)
   test_big_endian(IS_BIGENDIAN)
   if (IS_BIGENDIAN)
     add_compile_definitions(FLANG_BIG_ENDIAN=1)