[PATCH] Add readability-duplicate-include check to clang-tidy

[Richard]

================
Comment at: clang-tidy/readability/DuplicateIncludeCheck.cpp:62
@@ +61,3 @@
+    StringRef SearchPath, StringRef RelativePath, const Module *Imported) {
+  if (!SM_.isInMainFile(HashLoc)) {
+    return;
----------------
alexfh wrote:
> LegalizeAdulthood wrote:
> > alexfh wrote:
> > > What's the reason to limit the check to the main file only? I think, it should work on all headers as well. Also, sometimes it's fine to have duplicate includes even without macro definitions in between, e.g. when these #includes are in different namespaces.
> > > 
> > > I'd suggest using the same technique as in the IncludeOrderCheck: for each file collect all preprocessor directives sorted by SourceLocation. Then detect #include blocks (not necessarily the same way as its done in the IncludeOrderCheck. Maybe use the presense of any non-comment tokens between #includes as a boundary of blocks), and detect duplicate includes in each block.
> > If I remove the `isInMainFile`, how do I ensure that I don't attempt to modify system headers?
> Using `SourceLocation::isInSystemHeader()`.
Thanks, I'll try that.  The next question that brings to mind is how do I distinguish between headers that I own and headers used from third party libraries?

For instance, suppose I run a check on a clang refactoring tool and it uses `isInSystemHeader` and starts flagging issues in the clang tooling library headers.

The `compile_commands.json` doesn't contain any information about headers in my project, only translation units in my build, so it doesn't know whether or not included headers belong to me or third-party libraries.

http://reviews.llvm.org/D7982

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