[clang] Parse warning-suppression-mapping after setting up diagengine (llvm#125714)

We can emit diagnostics while parsing warning-suppression-mapping, make
sure command line flags take affect when emitting those.

(cherry picked from commit ecb016a)

Author: kadircet

clang/lib/Basic/Warnings.cpp

@@ -73,16 +73,6 @@ void clang::ProcessWarningOptions(DiagnosticsEngine &Diags,
   else
     Diags.setExtensionHandlingBehavior(diag::Severity::Ignored);
 
-  if (!Opts.DiagnosticSuppressionMappingsFile.empty()) {
-    if (auto FileContents =
-            VFS.getBufferForFile(Opts.DiagnosticSuppressionMappingsFile)) {
-      Diags.setDiagSuppressionMapping(**FileContents);
-    } else if (ReportDiags) {
-      Diags.Report(diag::err_drv_no_such_file)
-          << Opts.DiagnosticSuppressionMappingsFile;
-    }
-  }
-
   SmallVector<diag::kind, 10> _Diags;
   const IntrusiveRefCntPtr< DiagnosticIDs > DiagIDs =
     Diags.getDiagnosticIDs();
@@ -237,4 +227,17 @@ void clang::ProcessWarningOptions(DiagnosticsEngine &Diags,
       }
     }
   }
+
+  // Process suppression mappings file after processing other warning flags
+  // (like -Wno-unknown-warning-option) as we can emit extra warnings during
+  // processing.
+  if (!Opts.DiagnosticSuppressionMappingsFile.empty()) {
+    if (auto FileContents =
+            VFS.getBufferForFile(Opts.DiagnosticSuppressionMappingsFile)) {
+      Diags.setDiagSuppressionMapping(**FileContents);
+    } else if (ReportDiags) {
+      Diags.Report(diag::err_drv_no_such_file)
+          << Opts.DiagnosticSuppressionMappingsFile;
+    }
+  }
 }

clang/unittests/Basic/DiagnosticTest.cpp

@@ -346,4 +346,13 @@ TEST_F(SuppressionMappingTest, IsIgnored) {
   EXPECT_FALSE(Diags.isIgnored(diag::warn_unused_function,
                                SM.getLocForStartOfFile(ClangID)));
 }
+
+TEST_F(SuppressionMappingTest, ParsingRespectsOtherWarningOpts) {
+  Diags.getDiagnosticOptions().DiagnosticSuppressionMappingsFile = "foo.txt";
+  FS->addFile("foo.txt", /*ModificationTime=*/{},
+              llvm::MemoryBuffer::getMemBuffer("[non-existing-warning]"));
+  Diags.getDiagnosticOptions().Warnings.push_back("no-unknown-warning-option");
+  clang::ProcessWarningOptions(Diags, Diags.getDiagnosticOptions(), *FS);
+  EXPECT_THAT(diags(), IsEmpty());
+}
 } // namespace