Fixes breaking of string literals.

1. We now ignore all non-default string literals, including raw
literals.
2. We do not break inside escape sequences any more.

FIXME: We still break in trigraphs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176710 91177308-0d34-0410-b5e6-96231b3b80d8