Bring ASTReader and Writer into sync for the case where a canonical template...

Bring ASTReader and Writer into sync for the case where a canonical template specialization was written, which is non-canonical at the time of reading: force the reading of the ClassTemplateDecl if it was written.
The easiest way out is to store whether the decl was canonical at the time of writing.
Add test.

llvm-svn: 164927