Rename a test case.

tests/valid/int64.m:
    Rename this test case to unescaped_special_char since its current name
    conflicts with an upcoming standard library addition.  The new name
    is more descriptive anyway.

    Rename the type int64/0 defined in this test since an upcoming change
    will make that type a builtin (and hence a reserved name).

tests/valid/Mmakefile:
    Conform to the above change.

tests/valid/Mmakefile

@@ -148,7 +148,6 @@ OTHER_PROGS = \
 	inst_perf_bug_1 \
 	inst_perf_bug_2 \
 	instmap_generic_failure \
-	int64 \
 	lambda_inference\
 	lambda_instmap_bug \
 	lambda_instmap_bug2 \
@@ -244,6 +243,7 @@ OTHER_PROGS = \
 	unbound_inst_var \
 	unbound_tvar_in_lambda \
 	undead_proc \
+	unescaped_special_char \
 	unify_dummy \
 	uniq_mode_inf_bug \
 	uniq_unify \

tests/valid/unescaped_special_char.m

@@ -6,17 +6,17 @@
 % for this example, because it didn't properly escape special characters
 % such as \ in some of the C strings that it generated.
 
-:- module int64.
+:- module unescaped_special_char.
 
 :- interface.
 
-:- type int64.
+:- type myint64.
 
-:- func int64 /\ int64 = int64.
+:- func myint64 /\ myint64 = myint64.
 
 :- implementation.
 
-:- type int64 == int.
+:- type myint64 == int.
 
 :- pragma foreign_proc("C",
     (A::in) /\ (B::in) = (C::out),