Add diag::List::add_error() helper

Refactors a common pattern in the tint codebase.

Change-Id: Ia8a70d952fd8c204facd0120f24e43ccc9305622
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38840
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/reader/spirv/parser.cc

@@ -14,6 +14,8 @@
 
 #include "src/reader/spirv/parser.h"
 
+#include <utility>
+
 #include "src/reader/spirv/parser_impl.h"
 
 namespace tint {
@@ -30,10 +32,9 @@ bool Parser::Parse() {
   auto err_msg = impl_->error();
   if (!err_msg.empty()) {
     // TODO(bclayton): Migrate spirv::ParserImpl to using diagnostics.
-    diag::Diagnostic error{};
-    error.severity = diag::Severity::Error;
-    error.message = err_msg;
-    set_diagnostics({error});
+    diag::List diagnostics;
+    diagnostics.add_error(err_msg);
+    set_diagnostics(std::move(diagnostics));
   }
   return result;
 }

src/reader/spirv/parser.h

@@ -46,6 +46,7 @@ class Parser : public Reader {
 
  private:
   std::unique_ptr<ParserImpl> impl_;
+  Program program_;
 };
 
 }  // namespace spirv

src/reader/wgsl/parser_impl.cc

@@ -230,11 +230,7 @@ ParserImpl::Failure::Errored ParserImpl::add_error(const Token& t,
 ParserImpl::Failure::Errored ParserImpl::add_error(const Source& source,
                                                    const std::string& err) {
   if (silence_errors_ == 0) {
-    diag::Diagnostic diagnostic;
-    diagnostic.severity = diag::Severity::Error;
-    diagnostic.message = err;
-    diagnostic.source = source;
-    diags_.add(std::move(diagnostic));
+    diags_.add_error(err, source);
   }
   return Failure::kErrored;
 }