Build the Clang plugin on Windows.

tools/clang/plugins/FindBadConstructsConsumer.cpp

Index: tools/clang/plugins/FindBadConstructsConsumer.cpp
diff --git a/tools/clang/plugins/FindBadConstructsConsumer.cpp b/tools/clang/plugins/FindBadConstructsConsumer.cpp
index 770e2551d6e7c92f12032f850251ba5b8af738d1..4c40d52faae694983d4b18f5fdc8643bdf1eb6e7 100644
--- a/tools/clang/plugins/FindBadConstructsConsumer.cpp
+++ b/tools/clang/plugins/FindBadConstructsConsumer.cpp
@@ -559,8 +559,14 @@ FindBadConstructsConsumer::CheckRecordForRefcountIssue(
 // Adds either a warning or error, based on the current handling of
 // -Werror.
 DiagnosticsEngine::Level FindBadConstructsConsumer::getErrorLevel() {
+#if defined(LLVM_ON_WIN32)
+  // Only warn on Windows, since there are a lot of potential pre-existing
+  // issues.
+  return DiagnosticsEngine::Warning;
+#else
   return diagnostic().getWarningsAsErrors() ? DiagnosticsEngine::Error
                                             : DiagnosticsEngine::Warning;
+#endif
 }
 
 // Returns true if |base| specifies one of the Chromium reference counted