Add some comments to TypoCorrectionConsumer.

Also move the constructor for NamespaceSpecifierSet out of line to
improve the class' readability. I meant to do these two things while
cleaning up the previous TypoCorrectionConsumer changes and have them
folded into those changes.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210686 91177308-0d34-0410-b5e6-96231b3b80d8