Mark a few variables const. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220964 91177308-0d34-0410-b5e6-96231b3b80d8
Correct assert text from r220923

Noticed in post-commit review by Adrian Prantl.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220967 91177308-0d34-0410-b5e6-96231b3b80d8