[Option] For typo '-foo', suggest '--foo'

Summary:
https://reviews.llvm.org/rL321877 introduced the `OptTable::findNearest`
method, to find the closest edit distance option for a given string.
However, the implementation contained a bug: for a typo `-foo` with an
edit distance of 1 away from a valid option `--foo`, `findNearest`
would suggest a nearby option of `foo`. That is, the result would not
include the `--` prefix, and so was not a valid option.

Fix the bug by ensuring that the prefix string is initialized to one of
the valid prefixes for the option.

Test Plan: `check-llvm-unit`

Reviewers: v.g.vassilev, teemperor, ruiu, jroelofs, yamaguchi

Reviewed By: jroelofs

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41873

llvm-svn: 322109