C front-end: reject code containing duplicate labels

We previously silently accepted such code, maintaining the labels and
attaching them to multiple instructions. `goto` resolved to the first
such occurrence of the label.

GCC firmly rejects such code.

Author: tautschnig

regression/ansi-c/duplicate_label1/main.c

@@ -0,0 +1,9 @@
+int main()
+{
+  int x;
+label:
+  x = 1;
+  goto label;
+label:
+  x = 2;
+}

regression/ansi-c/duplicate_label1/test.desc

@@ -0,0 +1,9 @@
+CORE
+main.c
+
+^EXIT=(1|64)$
+^SIGNAL=0$
+error: duplicate label 'label'
+^CONVERSION ERROR$
+--
+^warning: ignoring

src/ansi-c/c_typecheck_code.cpp

@@ -505,7 +505,12 @@ void c_typecheck_baset::typecheck_for(codet &code)
 void c_typecheck_baset::typecheck_label(code_labelt &code)
 {
   // record the label
-  labels_defined[code.get_label()]=code.source_location();
+  if(!labels_defined.emplace(code.get_label(), code.source_location()).second)
+  {
+    error().source_location = code.source_location();
+    error() << "duplicate label '" << code.get_label() << "'" << eom;
+    throw 0;
+  }
 
   typecheck_code(code.code());
 }