Allow multi word types and typedef'd names as underlying enum types

Our previous attempt only worked with names like `short` or `unsigned`, but not
with e.g. `unsigned short` or `uint32_t`. This patch rectifies that.

Note that unlike clang we currently don't allow CV qualifiers for underlying
types. Since these are ignored by clang anyway, we don't expect to see too much
code in the wild making use of that feature however.

Author: danpoe

regression/ansi-c/enum_with_underlying_type/test.c

@@ -0,0 +1,19 @@
+typedef unsigned short USHORT;
+
+enum A : USHORT
+{
+  AV
+};
+enum B : unsigned short
+{
+  BV
+};
+enum C : signed long long
+{
+  CV
+};
+
+int main(void)
+{
+  return 0;
+}

regression/ansi-c/enum_with_underlying_type/test.desc

@@ -0,0 +1,12 @@
+CORE
+test.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^PARSING ERROR$
+--
+A previous attempt to allow underlying type specifications didn't allow typedef
+names or multi word types (e.g. unsigned int). We still don't actually fully
+apply the underlying type to the enum, but this at least lets us use parse code
+that uses this feature.

src/ansi-c/parser.y

@@ -1837,13 +1837,21 @@ enum_name:
         }
         ;
 
+basic_type_name_list:
+    basic_type_name
+  | basic_type_name_list basic_type_name
+
+enum_underlying_type:
+    basic_type_name_list
+  | typedef_name
+
 enum_underlying_type_opt:
         /* empty */
         {
           init($$);
           parser_stack($$).make_nil();
         }
-        | ':' basic_type_name
+        | ':' enum_underlying_type
         {
           $$=$2;
         }