Use several weighted factors to determine typo candidate viablity.

Use several weighted factors to determine typo candidate viablity.

Replace the simple Levenshtein edit distance for typo correction
candidates--and the hacky way adding namespace qualifiers would affect
the edit distance--with a synthetic "edit distance" comprised of several
factors and their relative weights. This also allows the typo correction
callback object to convey more information about the viability of a
correction candidate than simply viable or not viable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150495 91177308-0d34-0410-b5e6-96231b3b80d8