Introduce a getAsRange helper.

This requires making an error message a bit more generic, but that seems
a reasonable tradeoff.

Extracted from r260488 but simplified a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260967 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed