Remove uninstantiable constructor from DeclTemplate.h

Patch by Kim Gräsman!

Laszlo Nagy and me both ran into the same issue with SpecIterator from DeclTemplate.h causing an error:

tools/clang/include/clang/AST/DeclTemplate.h:560:22: error:
      no matching constructor for initialization of 'SetIteratorType' (aka
      'FoldingSetVectorIterator<clang::FunctionTemplateSpecializationInfo,
      typename SmallVector<FunctionTemplateSpecializationInfo *, 8>::iterator>')
    SpecIterator() : SetIter() {}

(see e.g. http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-November/033642.html)

The problem is the SpecIterator default constructor which needs a non-existent SetIter default constructor. SpecIterator's default constructor is evidently dead code as any call to it would fail with the above error.

This patch removes it.

Differential Revision: http://llvm-reviews.chandlerc.com/D2246