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

llvm-svn: 359685