icu_init.cpp is generated by generate_icu_init_cpp.py. Unfortunately, this means any C++ source code in that Python script is not checked by lint, clang-format, nor by error_codes.py. In fact, we already have a duplicate fassert code in our codebase due to this fact.

To fix this, we should move the static (unchanging) MONGO_INITIALIZER to a new static icu_init.cpp source file, and reserve the generated source file only to define an extern char array for the binary ICU data. We should create a header file as well to declare the extern'd symbol. This will avoid having duplicate fassert codes in the future.