[clang] [clang-tools-extra] [clang-tidy] fix misc-const-correctnes false-positive for fold expressions (PR #78320)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

I fixed the clang-tidy test case by separating them and I have added a test case to check that using a variable in the pack side of the fold expression would be detected (that did not change with this patch though, just as a sanity check).

https://github.com/llvm/llvm-project/pull/78320