InternPool: add representation for value of empty enums and unions

This is a bit odd, because this value doesn't actually exist:
see #15909. This gets all the empty enum/union behavior tests passing.

Also adds an assertion to `Sema.analyzeBodyInner` which would have
helped figure out the issue here much more quickly.

src/codegen/c.zig

@@ -946,6 +946,7 @@ pub const DeclGen = struct {
             .extern_func,
             .func,
             .enum_literal,
+            .empty_enum_value,
             => unreachable, // non-runtime values
             .int => |int| switch (int.storage) {
                 .u64, .i64, .big_int => try writer.print("{}", .{try dg.fmtIntLiteral(ty, val, location)}),

src/codegen/llvm.zig

@@ -3246,6 +3246,7 @@ pub const DeclGen = struct {
             },
             .variable,
             .enum_literal,
+            .empty_enum_value,
             => unreachable, // non-runtime values
             .extern_func, .func => {
                 const fn_decl_index = switch (val_key) {

src/codegen/spirv.zig

@@ -660,6 +660,7 @@ pub const DeclGen = struct {
                 .extern_func,
                 .func,
                 .enum_literal,
+                .empty_enum_value,
                 => unreachable, // non-runtime values
                 .int => try self.addInt(ty, val),
                 .err => |err| {