Fix enum bit-field ICE #1244

This previously resulted in an ICE:
cc65: Check failed: (Entry->Type->C & T_MASK_SIGN) == T_SIGN_SIGNED,
file 'cc65/symtab.c', line 874

This CHECK is in the code that deals with changing `int` bitfields to
`unsigned int`.

Work around this by treating enum bit-fields as having their signedness
specified, so this type change code does not get called.

Fixes #1244.

src/cc65/declare.c

@@ -1446,6 +1446,12 @@ static void ParseTypeSpec (DeclSpec* D, long Default, TypeCode Qualifiers,
             D->Type[0].C |= T_ENUM;
             SetESUSymEntry (D->Type, Entry);
             D->Type[1].C = T_END;
+            /* The signedness of enums is determined by the type, so say this is specified to avoid
+            ** the int -> unsigned int handling for plain int bit-fields in AddBitField.
+            */
+            if (SignednessSpecified) {
+                *SignednessSpecified = 1;
+            }
             break;
 
         case TOK_IDENT:

src/cc65/symtab.c

@@ -870,6 +870,7 @@ SymEntry* AddBitField (const char* Name, const Type* T, unsigned Offs,
         if (!SignednessSpecified) {
             /* int is treated as signed int everywhere except bit-fields; switch it to unsigned,
             ** since this is allowed for bit-fields and avoids sign-extension, so is much faster.
+            ** enums set SignednessSpecified to 1 to avoid this adjustment.
             */
             CHECK ((Entry->Type->C & T_MASK_SIGN) == T_SIGN_SIGNED);
             Entry->Type->C &= ~T_MASK_SIGN;