[SemaCXX] Fix false positive of -Wuninitialized-const-reference in empty function body.

Summary:
Some libraries use empty function to ignore unused variable warnings, which gets a new warning from `-Wuninitialized-const-reference`, discussed here https://reviews.llvm.org/D79895#2107604.
This patch should fix that.

Reviewers: hans, nick, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: aaron.ballman, riccibruno, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82425