Add a warning for using the same include guard in 2 different files

[llvmbot]

Bugzilla Link	28851
Version	unspecified
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@seanm

Extended Description

Imagine the following scenario:

// foo.cpp
#include "foo.h"

// foo.h
#ifndef FOO_H
#define FOO_H

#include "bar.h"

#endif

// bar.h
#ifndef FOO_H
#define FOO_H

#endif

This happens occasionally as a result of copy-pasting one header file to another header file to get the basic layout (header file premable, include guards, namespace layout, etc) in place and then fixing up. But occasionally one forgets to fix the header guard, leading to mysterious compiler errors.

It would be nice if this warned, by keeping for each translation unit a mapping of hashed include guard to include file name, and if an entry is found that exists in the hash table under a different filename, generating a warning.

[llvmbot]

assigned to @Weverything