Fix OptTable::findNearest() adding delimiter for free

Fix OptTable::findNearest() adding delimiter for free

Prior to this, OptTable::findNearest() thought that the input `--foo`
had an editing distance of 0 from an existing flag `--foo=`, which made
it suggest flags with delimiters more often than flags without one.
After this, it correctly assigns this case an editing distance of 1.

Differential Revision: https://reviews.llvm.org/D61373

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359685 91177308-0d34-0410-b5e6-96231b3b80d8