[Option] Fix PR37006 prefix choice in findNearest

Summary:
In https://bugs.llvm.org/show_bug.cgi?id=37006 Nico Weber points out a
flaw in `OptTable::findNearest`: if an option "foo"'s prefixes are "--"
and "-", then the nearest option for "--fob" will be "-foo". This is
incorrect, however, since the function is expected to return "--foo".

The bug is due to a naive loop that attempts to predetermines which
prefix is best. Instead, compute the edit distance for each prefix/name
pair.

Test Plan: `check-llvm`

Reviewers: thakis

Reviewed By: thakis

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46776

llvm-svn: 332299