tint/resolver: Consistently use utils::Result in ConstEval

Instead of using builder.Diagnostics().contains_errors()

Produces cleaner code and reduces scope of error handling.

Bug: tint:1661
Change-Id: I35af5ad1c6553f2cf74d1ce92dc14984f93b9db4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/102161
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/utils/result.h

@@ -17,6 +17,7 @@
 
 #include <ostream>
 #include <variant>
+
 #include "src/tint/debug.h"
 
 namespace tint::utils {
@@ -50,6 +51,20 @@ struct [[nodiscard]] Result {
     Result(const FAILURE_TYPE& failure)  // NOLINT(runtime/explicit):
         : value{failure} {}
 
+    /// Copy constructor with success / failure casting
+    /// @param other the Result to copy
+    template <typename S,
+              typename F,
+              typename = std::void_t<decltype(SUCCESS_TYPE{std::declval<S>()}),
+                                     decltype(FAILURE_TYPE{std::declval<F>()})>>
+    Result(const Result<S, F>& other) {  // NOLINT(runtime/explicit):
+        if (other) {
+            value = SUCCESS_TYPE{other.Get()};
+        } else {
+            value = FAILURE_TYPE{other.Failure()};
+        }
+    }
+
     /// @returns true if the result was a success
     operator bool() const {
         Validate();

src/tint/utils/result_test.cc

@@ -51,5 +51,17 @@ TEST(ResultTest, CustomFailure) {
     EXPECT_EQ(r.Failure(), "oh noes!");
 }
 
+TEST(ResultTest, ValueCast) {
+    struct X {};
+    struct Y : X {};
+
+    Y* y = nullptr;
+    auto r_y = Result<Y*>{y};
+    auto r_x = Result<X*>{r_y};
+
+    (void)r_x;
+    (void)r_y;
+}
+
 }  // namespace
 }  // namespace tint::utils