util/bitset: Make C++ wrapper trivially constructible.

In order to fix a build failure on compilers not implementing
unrestricted unions, which is a C++11 feature.

v2: Provide signed integer comparison and assignment operators instead
    of BITSET_WORD ones to avoid spurious ambiguity warnings on
    comparisons with a signed integer literal.

Fixes: ba79a90fb5 "glsl: Switch ast_type_qualifier to a 128-bit bitset."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105238
Tested-by: Roland Scheidegger <sroland@vmware.com>
Tested-By: George Kyriazis <george.kyriazis@intel.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/compiler/glsl/ast.h

@@ -477,8 +477,6 @@ struct ast_type_qualifier {
    DECLARE_BITSET_T(bitset_t, 128);
 
    union flags {
-      flags() : i(0) {}
-
       struct {
 	 unsigned invariant:1;
          unsigned precise:1;

src/compiler/glsl/glsl_parser.yy

@@ -96,7 +96,6 @@ static bool match_layout_qualifier(const char *s1, const char *s2,
 %parse-param {struct _mesa_glsl_parse_state *state}
 
 %union {
-   YYSTYPE() {}
    int n;
    int64_t n64;
    float real;