Sema: improve behavior of comptime_int backed enums

test/behavior/enum.zig

@@ -1175,3 +1175,10 @@ test "Non-exhaustive enum with nonstandard int size behaves correctly" {
     const E = enum(u15) { _ };
     try expect(@sizeOf(E) == @sizeOf(u15));
 }
+
+test "Non-exhaustive enum backed by comptime_int" {
+    const E = enum(comptime_int) { a, b, c, _ };
+    comptime var e: E = .a;
+    e = @intToEnum(E, 378089457309184723749);
+    try expect(@enumToInt(e) == 378089457309184723749);
+}

test/behavior/union.zig

@@ -690,7 +690,7 @@ test "union with only 1 field casted to its enum type which has enum value speci
 
     var e = Expr{ .Literal = Literal{ .Bool = true } };
     comptime try expect(Tag(ExprTag) == comptime_int);
-    var t = @as(ExprTag, e);
+    comptime var t = @as(ExprTag, e);
     try expect(t == Expr.Literal);
     try expect(@enumToInt(t) == 33);
     comptime try expect(@enumToInt(t) == 33);

test/cases/compile_errors/enum_backed_by_comptime_int_must_be_comptime.zig

@@ -0,0 +1,11 @@
+pub export fn entry() void {
+    const E = enum(comptime_int) { a, b, c, _ };
+    var e: E = .a;
+    _ = e;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:12: error: variable of type 'tmp.entry.E' must be const or comptime