[SemaTemplate] Fix uncorrected typos after pack expansion

Summary:
This case is particularly important for clangd, as it is triggered after
inserting the snippet for variadic functions.

Reviewers: kadircet, ilya-biryukov

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64677

llvm-svn: 366200