GN: Check if targets with precompiled headers use unsupported toolchains.

tools/gn/source_file_type.h

Index: tools/gn/source_file_type.h
diff --git a/tools/gn/source_file_type.h b/tools/gn/source_file_type.h
index c43b4324443e5184709390fd5bfb658fc6a5edfd..af71d1c99ebb139322a8eeefd0767ce32901b8b8 100644
--- a/tools/gn/source_file_type.h
+++ b/tools/gn/source_file_type.h
@@ -28,4 +28,19 @@ enum SourceFileType {
 
 SourceFileType GetSourceFileType(const SourceFile& file);
 
+// Represents a set of tool types.
+class SourceFileTypeSet {
+ public:
+  SourceFileTypeSet() {
+    for (auto& flag : flags_)
+      flag = 0;
+  }
+
+  void Set(SourceFileType type) { flags_[static_cast<int>(type)] = true; }
+  bool Get(SourceFileType type) const { return flags_[static_cast<int>(type)]; }
+
+ private:
+  bool flags_[static_cast<int>(SOURCE_NUMTYPES)];
+};
+
 #endif  // TOOLS_GN_SOURCE_FILE_TYPE_H_