Store first and last newline position in the token text for string literals and comments.

Summary:
Store first and last newline position in the token text for string literals and
comments to avoid doing .find('\n') for each possible solution.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1556

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189758 91177308-0d34-0410-b5e6-96231b3b80d8