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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164927 91177308-0d34-0410-b5e6-96231b3b80d8