[Lexer] Finding beginning of token with escaped new line

Summary:
Lexer::GetBeginningOfToken produced invalid location when
backtracking across escaped new lines.

This fixes PR26228

Reviewers: akyrtzi, alexfh, rsmith, doug.gregor

Reviewed By: alexfh

Subscribers: alexfh, cfe-commits

Patch by Paweł Żukowski!

Differential Revision: https://reviews.llvm.org/D30748

llvm-svn: 310576