[clang][ASTImporter] Import TemplateName correctly

This is a fix for a problem when multiple template
specializations are created by ASTImporter for the
same specialization. The problem happens if a
TemplateName is imported that points to a template
delcaration (for a template template argument)
(specialization) that has multiple instances in the
declaration chain. If two TemplateName objects contain
different pointers to a template specialization,
these TemplateName objects will have different checksum
even if they point into the same declaration chain.
The problem is fixed if the canonical declaration is used.

Reviewed By: vabridgers, donat.nagy

Differential Revision: https://reviews.llvm.org/D144622