Merging r292034: (PR32315)

Tweaked to not remove the non-const variants as to not change the ABI.

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r292034 | majnemer | 2017-01-14 13:54:58 -0800 (Sat, 14 Jan 2017) | 7 lines

Adding const overloads of operator* and operator-> for DenseSet iterators

This fixes some problems when building ClangDiagnostics.cpp on Visual Studio 2017 RC. As far as I understand, there was a change in the implementation of the constructor for std::vector with two iterator parameters, which in our case causes an attempt to dereference const Iterator objects. Since there was no overload for a const Iterator, the compile would fail.

Patch by Hugo Puhlmann!

Differential Revision: https://reviews.llvm.org/D28726
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llvm-svn: 301479