C front-end: tolerate type differences with asm renaming

The example found in glibc shows how asm renaming may be combined with
changes in return type names (even when there isn't actually a bit-level
difference amongst the types). Weakening the asm-renamed declaration by
marking it "incomplete" works around this problem.

Author: tautschnig

regression/ansi-c/asm4/main.c

@@ -0,0 +1,26 @@
+// from Fedora's glibc
+struct dirent
+{
+  int dummy;
+};
+
+struct dirent64
+{
+  int dummy;
+};
+
+#ifdef __GNUC__
+extern struct dirent *readdir(void *__dirp) __asm__(
+  ""
+  "readdir64");
+extern struct dirent64 *readdir64(void *__dirp);
+#endif
+
+int main()
+{
+#ifdef __GNUC__
+  int x;
+  (void)readdir(&x);
+#endif
+  return 0;
+}

regression/ansi-c/asm4/test.desc

@@ -0,0 +1,8 @@
+CORE test-c++-front-end
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/c_typecheck_base.cpp

@@ -668,6 +668,10 @@ void c_typecheck_baset::apply_asm_label(
   {
     symbol.name=asm_label;
     symbol.base_name=asm_label;
+    // asm renaming may be combined with varied return types - make sure the
+    // actual definition sets the final type
+    if(symbol.type.id() == ID_code)
+      symbol.type.set(ID_C_incomplete, true);
   }
 
   if(symbol.name!=orig_name)