tccgen: scope levels for local symbols

... for fast redeclaration checks

Also, check function parameters too:
    void foo(int a) { int a; ... }

Also, try to fix struct/union/enum's on different scopes:
    { struct xxx { int x; };
         { struct xxx { int y; }; ... }}
and some (probably not all) combination with incomplete
declarations "struct xxx;"

Replaces 2bfedb1867
and 07d896c8e5

Fixes cf95ac399c

tcc.h

@@ -459,7 +459,10 @@ typedef struct AttributeDef {
 /* symbol management */
 typedef struct Sym {
     int v;    /* symbol token */
-    int asm_label;    /* associated asm label */
+    union {
+        int asm_label;    /* associated asm label */
+        int scope;  /* scope level for locals */
+    };
     union {
         long r;    /* associated register */
         struct Attribute a;
@@ -707,7 +710,6 @@ struct TCCState {
 				   Liuux 2.4.26 can't find initrd when compiled with a new algorithm */
     int dollars_in_identifiers;	/* allows '$' char in indentifiers */
     int normalize_inc_dirs;	/* remove non-existent or duplicate directories from include paths */
-    int no_type_redef_check;    /* no local vars redefinition check */
 
     /* warning switches */
     int warn_write_strings;