Allow MSBuildWarningsAsMessages with comma separation (#11522)

Fixes #7094

Context
We don't support NAT011,NAT012, when warnings with comma separation, the Warings are not suppressed.

Changes Made
Parse warning codes with comma separation and semicolon

Testing
MSBuildWarningsAsMessagesWithCommaSeparation()

Author: JaynieBai

src/Build.UnitTests/WarningsAsMessagesAndErrors_Tests.cs

@@ -527,5 +527,33 @@ public void TaskReturnsFailureButDoesNotLogError_ContinueOnError_False()
                 logger.AssertLogContains("MSB4181");
             }
         }
+
+        /// <summary>
+        /// MSBuildWarningsAsMessages should allow comma separation.
+        /// </summary>
+        [Fact]
+        public void MSBuildWarningsAsMessagesWithCommaSeparation()
+        {
+            using (TestEnvironment env = TestEnvironment.Create(_output))
+            {
+                var content = """
+                <Project>
+                    <PropertyGroup>
+                       <MSBuildWarningsAsMessages>NAT011,NAT012</MSBuildWarningsAsMessages>
+                    </PropertyGroup>
+
+                    <Target Name='Build'>
+                        <Warning Code="NAT011" Text="You fail" />
+                        <Warning Code="NAT012" Text="Other Fail" />
+                    </Target>
+                </Project>
+                """;
+                TransientTestProjectWithFiles proj = env.CreateTestProjectWithFiles(content);
+
+                MockLogger logger = proj.BuildProjectExpectSuccess();
+                logger.WarningCount.ShouldBe(0);
+                logger.ErrorCount.ShouldBe(0);
+            }
+        }
     }
 }

src/Build/BackEnd/Components/RequestBuilder/RequestBuilder.cs

@@ -1520,7 +1520,9 @@ private static ISet<string> ParseWarningCodes(string warnings)
                 return null;
             }
 
-            return new HashSet<string>(ExpressionShredder.SplitSemiColonSeparatedList(warnings), StringComparer.OrdinalIgnoreCase);
+            return new HashSet<string>(ExpressionShredder.SplitSemiColonSeparatedList(warnings)
+            .SelectMany(w => w.Split([','], StringSplitOptions.RemoveEmptyEntries))
+            .Select(w => w.Trim()), StringComparer.OrdinalIgnoreCase);
         }
 
         private sealed class DedicatedThreadsTaskScheduler : TaskScheduler