Bytecode Interpreter: Use in `Expr::tryEvaluateStrLen`

[tbaederr]

We need an equivalent of EvaluateBuiltinStrLen, and use that in Expr::tryEvaluateStrLen as well as Expr::tryEvaluateString.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Timm Baeder (tbaederr)

We need an equivalent of `EvaluateBuiltinStrLen`, and use that in `Expr::tryEvaluateStrLen` as well as `Expr::tryEvaluateString`.