tint: Make uniformity analysis failures a hard error

These have been warnings for multiple months.
Time to properly turn this on.

Bug: tint:880
Change-Id: I3b38f672309b5acd48c12a38dc5a1675f3c62470
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/103480
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>

src/tint/resolver/resolver.cc

@@ -167,7 +167,7 @@ bool Resolver::ResolveInternal() {
 
     if (!enabled_extensions_.Contains(ast::Extension::kChromiumDisableUniformityAnalysis)) {
         if (!AnalyzeUniformity(builder_, dependencies_)) {
-            // TODO(jrprice): Reject programs that fail uniformity analysis.
+            return false;
         }
     }
 

src/tint/resolver/uniformity.cc

@@ -1548,9 +1548,8 @@ class UniformityGraph {
         // Helper to produce a diagnostic message with the severity required by this invocation of
         // the `MakeError` function.
         auto report = [&](Source source, std::string msg) {
-            // TODO(jrprice): Switch to error instead of warning when feedback has settled.
             diag::Diagnostic error{};
-            error.severity = note ? diag::Severity::Note : diag::Severity::Warning;
+            error.severity = note ? diag::Severity::Note : diag::Severity::Error;
             error.system = diag::System::Resolver;
             error.source = source;
             error.message = msg;