[clang-rename] Add the USR of incomplete decl to the USRSet.

Summary:
This fixes a clangd rename issue, which is missing the reference of
an incomplete specialization.

Unfortunately, I didn't reproduce this issue in clang-rename, I guess
the input `FoundDecl` of AdditionalUSRFinder is different in clangd vs
clang-rename, clang-rename uses the underlying CXXRecordDecl of the
ClassTemplateDecl, which is fixed in https://github.com/llvm/llvm-project/commit/5d862c042b52ae2aad37471d0b83b6c678a520e3;
while clangd-rename uses the ClassTemplateDecl.

Reviewers: kbobyrev

Reviewed By: kbobyrev

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74829