add C23 error diagnostic: missing type specifier

src/Diagnostics.zig

@@ -374,6 +374,10 @@ const messages = struct {
         const kind = .warning;
         const all = true;
     };
+    pub const missing_type_specifier_c2x = struct {
+        const msg = "a type specifier is required for all declarations";
+        const kind = .@"error";
+    };
     pub const multiple_storage_class = struct {
         const msg = "cannot combine with previous '{s}' declaration specifier";
         const extra = .str;

src/Type.zig

@@ -1727,7 +1727,11 @@ pub const Builder = struct {
                     ty = typeof;
                 } else {
                     ty.specifier = .int;
-                    try p.err(.missing_type_specifier);
+                    if (p.comp.langopts.standard.atLeast(.c2x)) {
+                        try p.err(.missing_type_specifier_c2x);
+                    } else {
+                        try p.err(.missing_type_specifier);
+                    }
                 }
             },
             .void => ty.specifier = .void,

test/cases/missing type specifier.c

@@ -0,0 +1,4 @@
+//aro-args
+#define EXPECTED_ERRORS \
+    "missing type specifier.c:4:8: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]"
+static x = 5;

test/cases/missing type specifier_c2x.c

@@ -0,0 +1,4 @@
+//aro-args -std=c2x
+#define EXPECTED_ERRORS \
+    "missing type specifier_c2x.c:4:8: error: a type specifier is required for all declarations"
+static x = 5;