[clang-tidy] Added -system-headers option.

Added -system-headers option to allow display of warnings from system headers.
This is needed for testing libcxx, for example.

llvm-svn: 220826
diff --git a/clang-tools-extra/clang-tidy/ClangTidyOptions.cpp b/clang-tools-extra/clang-tidy/ClangTidyOptions.cpp
index 6a3aaeb..acdcb4d 100644
--- a/clang-tools-extra/clang-tidy/ClangTidyOptions.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidyOptions.cpp
@@ -101,6 +101,7 @@
   ClangTidyOptions Options;
   Options.Checks = "";
   Options.HeaderFilterRegex = "";
+  Options.SystemHeaders = false;
   Options.AnalyzeTemporaryDtors = false;
   Options.User = llvm::None;
   for (ClangTidyModuleRegistry::iterator I = ClangTidyModuleRegistry::begin(),
@@ -122,6 +123,8 @@
 
   if (Other.HeaderFilterRegex)
     Result.HeaderFilterRegex = Other.HeaderFilterRegex;
+  if (Other.SystemHeaders)
+    Result.SystemHeaders = Other.SystemHeaders;
   if (Other.AnalyzeTemporaryDtors)
     Result.AnalyzeTemporaryDtors = Other.AnalyzeTemporaryDtors;
   if (Other.User)