Added ClassTemplateSpecializationDecl, which is a subclass of

Added ClassTemplateSpecializationDecl, which is a subclass of
CXXRecordDecl that is used to represent class template
specializations. These are canonical declarations that can refer to
either an actual class template specialization in the code, e.g.,

  template<> class vector<bool> { };

or to a template instantiation. However, neither of these features is
actually implemented yet, so really we're just using (and uniqing) the
declarations to make sure that, e.g., A<int> is a different type from
A<float>. Note that we carefully distinguish between what the user
wrote in the source code (e.g., "A<FLOAT>") and the semantic entity it
represents (e.g., "A<float, int>"); the former is in the sugared Type,
the latter is an actual Decl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64716 91177308-0d34-0410-b5e6-96231b3b80d8