Get pragma foreign_type working for the C backend.

Estimated hours taken: 16
Branches: main

Get pragma foreign_type working for the C backend.

doc/reference_manual.texi:
    Document C pragma foreign_types.

compiler/prog_data.m:
    Add il_foreign_type and c_foreign_type which contain all the
    necessary data to output a foreign_type on the respective backends.
    Change foreign_language_type to refer to these new types.

compiler/prog_io_pragma.m:
    Handle the changes to foreign_language_type, and parse C
    foreign_type declarations.

compiler/hlds_data.m:
    Change the hlds_data__foreign_type type so that it records both the
    C and IL foreign types.  This will allow one to output both foreign
    type declarations when doing intermodule optimization.

compiler/make_hlds.m:
    Changes so that we store both the IL and C foreign types in
    hlds_data__foreign_type.
    Also add an error checking pass where we check that there is a
    foreign type for the back-end we are currently compiling to.

compiler/foreign.m:
    Change to_exported_type so that it works for both the C and IL
    backends by getting either the C or IL foreign_type definition.

compiler/llds.m:
compiler/pragma_c_gen.m:
    Change pragma_c_input and pragma_c_output so that they record
    whether or not a type is a foreign_type and if so what is the string
    which represents that foreign_type.

compiler/llds_out.m:
    When outputting pragma c_code variables that represent foreign_types
    get the casts correct.  Note that this adds the constraint on C
    foreign types that they are word sized, as all we are doing is
    casts, not boxing and unboxing.

compiler/mlds.m:
    Change mlds__foreign_type so that we store whether a type is an IL
    type or a C type.  It is the responsibility of the code generator
    that we never create a reference to a IL foreign type when on the C
    back-end, and vice versa.

compiler/mercury_to_mercury.m:
    Handle changes to prog_data__foreign_type.

compiler/hlds_out.m:
compiler/intermod.m:
compiler/magic_util.m:
compiler/ml_code_gen.m:
compiler/ml_type_gen.m:
compiler/recompilation_usage.m:
compiler/term_util.m:
compiler/type_ctor_info.m:
compiler/unify_proc.m:
    Handle changes to hlds_data__foreign_type.

compiler/exprn_aux.m:
compiler/livemap.m:
compiler/middle_rec.m:
compiler/opt_util.m:
    Handle changes to the pragma_c_input and pragma_c_output types.

compiler/ml_code_util.m:
compiler/mlds_to_gcc.m:
compiler/mlds_to_il.m:
compiler/mlds_to_java.m:
compiler/mlds_to_c.m:
    Handle changes to mlds__foreign_type.

compiler/mlds_to_java.m

@@ -1251,7 +1251,7 @@ get_java_type_initializer(mlds__native_bool_type) = "false".
 get_java_type_initializer(mlds__native_int_type) = "0".
 get_java_type_initializer(mlds__native_float_type) = "0".
 get_java_type_initializer(mlds__native_char_type) = "0".
-get_java_type_initializer(mlds__foreign_type(_, _, _)) = _ :-
+get_java_type_initializer(mlds__foreign_type(_)) = _ :-
 	unexpected(this_file, 
 		"get_type_initializer: variable has foreign_type"). 
 get_java_type_initializer(mlds__class_type(_, _, _)) = "null".
@@ -1619,7 +1619,7 @@ output_type(mlds__native_int_type)   --> io__write_string("int").
 output_type(mlds__native_float_type) --> io__write_string("double").
 output_type(mlds__native_bool_type) --> io__write_string("boolean").
 output_type(mlds__native_char_type)  --> io__write_string("char").
-output_type(mlds__foreign_type(_, _, _))  -->
+output_type(mlds__foreign_type(_))  -->
 	{ unexpected(this_file, "output_type: foreign_type NYI.") }.
 output_type(mlds__class_type(Name, Arity, ClassKind)) -->
 	( { ClassKind = mlds__enum } ->