IWYU: Define a macro to tell code it is preprocessing for iwyu

There are some cases where satisfying IWYU breaks compilation, such as
forward-declaring the `std::hash<>` template with C++ standard libraries
that use an inline namespace inside `std`.  Define a macro during
include-what-you-use preprocessing so that code can adapt.

CMakeLists.txt

@@ -285,7 +285,7 @@ if(CMake_RUN_IWYU)
     message(FATAL_ERROR "CMake_RUN_IWYU is ON but include-what-you-use is not found!")
   endif()
   set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE
-    "${IWYU_COMMAND};-Xiwyu;--mapping_file=${CMake_SOURCE_DIR}/Utilities/IWYU/mapping.imp;-w")
+    "${IWYU_COMMAND};-Xiwyu;--mapping_file=${CMake_SOURCE_DIR}/Utilities/IWYU/mapping.imp;-w;-DCMAKE_IWYU")
 endif()