C front-end: ignore or reject conflicting function redeclaration

A declaration of a function currently being type checked should not
replace the existing declaration. To accomplish this, make sure a
definition `type name() { ... }` fixes the type of `name` to have no
parameters and no ellipsis. This, more precise, typing highlighted a bug
in the goto-harness/parameter_and_global_variable_distinction regression
test (and the underlying implementation!), which assumed that the type
of `int (*fptr_t)(void)` did not match `int call_f()`.

Test generated by C-Reduce, starting from an SV-COMP task.

Author: tautschnig

regression/ansi-c/invalid_redeclaration1/main.c

@@ -0,0 +1,14 @@
+#ifdef same_return_type
+#  define T int
+#else
+#  define T struct b
+#endif
+
+int a()
+{
+  T a(c);
+}
+
+int main()
+{
+}

regression/ansi-c/invalid_redeclaration1/test-same_type.desc

@@ -0,0 +1,12 @@
+CORE
+main.c
+-Dsame_return_type
+^EXIT=0$
+^SIGNAL=0$
+--
+Invariant check failed
+^CONVERSION ERROR$
+--
+This previously triggered the following (undesired) invariant violation:
+File: ../src/goto-programs/goto_convert_functions.cpp:167 function: convert_function
+Condition: parameter identifier should not be empty

regression/ansi-c/invalid_redeclaration1/test.desc

@@ -0,0 +1,9 @@
+CORE test-c++-front-end
+main.c
+
+^EXIT=(64|1)$
+^SIGNAL=0$
+(function symbol 'a' redefined with a different type|symbol 'c' is unknown)
+^CONVERSION ERROR$
+--
+Invariant check failed

regression/cbmc/typedef-return-anon-struct1/test.desc

@@ -6,6 +6,6 @@ activate-multi-line-match
 ^EXIT=0$
 ^SIGNAL=0$
 Base name\.+: return'\nMode\.+: C\nType\.+: MYSTRUCT
-Base name\.+: fun\nMode\.+: C\nType\.+: MYSTRUCT \(\)
+Base name\.+: fun\nMode\.+: C\nType\.+: MYSTRUCT \(void\)
 --
 warning: ignoring

regression/cbmc/typedef-return-anon-union1/test.desc

@@ -5,6 +5,6 @@ main.c
 activate-multi-line-match
 ^EXIT=0$
 ^SIGNAL=0$
-Base name\.+: fun\nMode\.+: C\nType\.+: MYUNION \(\)
+Base name\.+: fun\nMode\.+: C\nType\.+: MYUNION \(void\)
 --
 warning: ignoring

regression/cbmc/typedef-return-struct1/test.desc

@@ -5,7 +5,7 @@ main.c
 activate-multi-line-match
 ^EXIT=0$
 ^SIGNAL=0$
-Base name\.+: fun\nMode\.+: C\nType\.+: struct tag_struct_name \(\)
-Base name\.+: fun2\nMode\.+: C\nType\.+: MYSTRUCT \(\)
+Base name\.+: fun\nMode\.+: C\nType\.+: struct tag_struct_name \(void\)
+Base name\.+: fun2\nMode\.+: C\nType\.+: MYSTRUCT \(void\)
 --
 warning: ignoring

regression/cbmc/typedef-return-type1/test.desc

@@ -5,7 +5,7 @@ main.c
 activate-multi-line-match
 ^EXIT=0$
 ^SIGNAL=0$
-Base name\.+: fun\nMode\.+: C\nType\.+: signed int \(\)
-Base name\.+: fun2\nMode\.+: C\nType\.+: MYINT \(\)
+Base name\.+: fun\nMode\.+: C\nType\.+: signed int \(void\)
+Base name\.+: fun2\nMode\.+: C\nType\.+: MYINT \(void\)
 --
 warning: ignoring

regression/cbmc/typedef-return-type2/test.desc

@@ -5,7 +5,7 @@ main.c
 activate-multi-line-match
 ^EXIT=0$
 ^SIGNAL=0$
-Base name\.+: fun\nMode\.+: C\nType\.+: MYINT \(\)
-Base name\.+: fun2\nMode\.+: C\nType\.+: ALTINT \(\)
+Base name\.+: fun\nMode\.+: C\nType\.+: MYINT \(void\)
+Base name\.+: fun2\nMode\.+: C\nType\.+: ALTINT \(void\)
 --
 warning: ignoring

regression/cbmc/typedef-return-type3/test.desc

@@ -3,8 +3,8 @@ main.c
 --show-symbol-table --function fun
 // Enable multi-line checking
 activate-multi-line-match
-Base name\.+: fun\nMode\.+: C\nType\.+: MYINT \(\)
-Base name\.+: fun2\nMode\.+: C\nType\.+: CHAINEDINT \(\)
+Base name\.+: fun\nMode\.+: C\nType\.+: MYINT \(void\)
+Base name\.+: fun2\nMode\.+: C\nType\.+: CHAINEDINT \(void\)
 ^EXIT=0$
 ^SIGNAL=0$
 --

regression/cbmc/typedef-return-union1/test.desc

@@ -5,7 +5,7 @@ main.c
 activate-multi-line-match
 ^EXIT=0$
 ^SIGNAL=0$
-Base name\.+: fun\nMode\.+: C\nType\.+: union tag_union_name \(\)
-Base name\.+: fun2\nMode\.+: C\nType\.+: MYUNION \(\)
+Base name\.+: fun\nMode\.+: C\nType\.+: union tag_union_name \(void\)
+Base name\.+: fun2\nMode\.+: C\nType\.+: MYUNION \(void\)
 --
 warning: ignoring