[clangd] Handle DeducedTemplateSpecializationType in TargetFinder

Summary:
This is a workaround for https://bugs.llvm.org/show_bug.cgi?id=42914.
Once that is fixed, the handling in VisitDeducedTyped() should be sufficient.

Fixes https://github.com/clangd/clangd/issues/242

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72119