[Support] Add case-insensitive versions of StringSwitch members.

This adds support for CaseLower, CasesLower, StartsWithLower, and
EndsWithLower.

Differential revision: https://reviews.llvm.org/D24686

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283244 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed