[ASTImporter] Import described template (if any) of function.

Summary:
When a function is imported, check if it has a described template.
The name lookup is corrected to find the templated entity in this case.
The described template of the function is imported too.

Reviewers: a.sidorin, a_sidorin

Reviewed By: a_sidorin

Subscribers: a_sidorin, martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D49235

llvm-svn: 337260