c: remove inline 'backing' type tag for enums

It was meant as a micro-optimization but actually keeping it in sync was
annoying. This for example was broken:

```
extern enum foo { ... } g_foo;
enum foo g_foo;
```

3 files changed, 4 insertions, 9 deletions

diff --git a/src/c.c b/src/c.c
index d6879ad..6446ab5 100644
--- a/src/c.c
+++ b/src/c.c
@@ -2198,7 +2198,6 @@ buildenum(CComp *cm, internstr name, const Span *span, int id)
       ty = completetype(name, id, &td);
    else
       ty = mktagtype(name, &td);
-   ty.backing = td.backing;
    return ty;
 }
 
diff --git a/src/c_type.c b/src/c_type.c
index bf24653..5853eb8 100644
--- a/src/c_type.c
+++ b/src/c_type.c
@@ -214,7 +214,7 @@ completetype(internstr name, int id, TypeData *td)
       assert(tagtypetags[id] == name && "bad redefn");
    else
       tagtypetags[id] = name;
-   return mktype(td->t, .dat = interntd(td), .backing = td->t == TYENUM ? td->backing : 0);
+   return mktype(td->t, .dat = interntd(td));
 }
 
 Type
diff --git a/src/c_type.h b/src/c_type.h
index ecca949..6f9a265 100644
--- a/src/c_type.h
+++ b/src/c_type.h
@@ -30,10 +30,7 @@ enum typeflagmask {
 typedef union Type {
    struct {
       uchar t; /* type tag */
-      union {
-         uchar flag;
-         uchar backing; /* type tag for enum backing int */
-      };
+      uchar flag;
       union {
          struct {
             uchar child; /* prim type tag */
@@ -141,11 +138,10 @@ Type cvtarith(Type a, Type b);
 static inline Type
 typechild(Type t)
 {
-   if (t.t == TYENUM) return mktype(t.backing ? t.backing : typedata[t.dat].backing);
+   if (t.t == TYENUM) return mktype(typedata[t.dat].backing);
    if (t.flag & TFCHLDPRIM) return mktype(t.child);
    if (t.flag & TFCHLDISDAT) {
       Type chld = mktype(typedata[t.dat].t, .dat = t.dat);
-      if (chld.t == TYENUM) chld.backing = typedata[t.dat].backing;
       return chld;
    }
    return typedata[t.dat].child;
@@ -153,7 +149,7 @@ typechild(Type t)
 static inline enum typetag
 scalartypet(Type t)
 {
-   if (t.t == TYENUM) return t.backing ? t.backing : typedata[t.dat].backing;
+   if (t.t == TYENUM) return typedata[t.dat].backing;
    if (isptrcvt(t)) return TYPTR;
    assert(!iscomplex(t));
    return t.t;