[clang] Fixing incorrect implicit deduction guides (PR41549)

Summary:
[[ https://bugs.llvm.org/show_bug.cgi?id=41549 | bug report ]]

Before this patch, implicit deduction guides were generated from the first declaration found by lookup.
With this patch implicit deduction guides are generated from the definition of the class template.
Also added test that was previously failing.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits, Quuxplusone

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63072

llvm-svn: 363361