[CMake] Fix Xcode build with LLVM_ENABLE_OBJLIB.

This amends chapuni's r246156 to handle an Xcode quirk, one even called out
in the CMake documentation:

    Some native build systems may not like targets that have only object files,
    so consider adding at least one real source file to any target that
    references $<TARGET_OBJECTS:objlib>.

I've limited the scope of this hack to Xcode for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247305 91177308-0d34-0410-b5e6-96231b3b80d8

Author: jrose-apple

cmake/dummy.cpp

@@ -0,0 +1 @@
+typedef int dummy;

cmake/modules/AddLLVM.cmake

@@ -590,6 +590,12 @@ macro(add_llvm_executable name)
 
   add_windows_version_resource_file(ALL_FILES ${ALL_FILES})
 
+  if(XCODE)
+    # Note: the dummy.cpp source file provides no definitions. However,
+    # it forces Xcode to properly link the static library.
+    list(APPEND ALL_FILES "${LLVM_SOURCE_DIR}/cmake/dummy.cpp")
+  endif()
+
   if( EXCLUDE_FROM_ALL )
     add_executable(${name} EXCLUDE_FROM_ALL ${ALL_FILES})
   else()