PR20473: Don't "deduplicate" string literals with the same value but different
lengths! In passing, simplify string literal deduplication by relying on LLVM
to deduplicate the underlying constant values.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214222 91177308-0d34-0410-b5e6-96231b3b80d8