C front-end: fix type checking with merged type

The changes of 4a50ac1 resulted in a branch still using symbol.type
when that wasn't necessarily the type being processed.

The additional test, which triggers this code path, was constructed
using creduce.

Author: tautschnig

regression/ansi-c/gcc_attributes1/main.c

@@ -39,6 +39,10 @@ void my_f1()
   while(1);
 }
 
+inline void a() __attribute__((noreturn))
+{
+}
+
 //
 // unused
 //

src/ansi-c/ansi_c_declaration.cpp

@@ -141,9 +141,9 @@ void ansi_c_declarationt::to_symbol(
   symbol.is_weak=get_is_weak();
 
   // is it a function?
-  const typet &type = symbol.type.id() == ID_merged_type
-                        ? to_merged_type(symbol.type).last_type()
-                        : symbol.type;
+  typet &type = symbol.type.id() == ID_merged_type
+                  ? to_merged_type(symbol.type).last_type()
+                  : symbol.type;
 
   if(type.id() == ID_code && !symbol.is_type)
   {
@@ -153,7 +153,7 @@ void ansi_c_declarationt::to_symbol(
     symbol.is_file_local=get_is_static();
 
     if(get_is_inline())
-      to_code_type(symbol.type).set_inlined(true);
+      to_code_type(type).set_inlined(true);
 
     if(
       config.ansi_c.mode == configt::ansi_ct::flavourt::GCC ||