[clang-scan-deps] Improve string/character literal skipping

[clang-scan-deps] Improve string/character literal skipping

The existing string/character literal skipping code in the
dependency directives source minimizer has two issues:

- It doesn't stop the scanning when a newline is reached before the terminating character,
unlike the lexer which considers the token to be done (even if it's invalid) at the end of the line.

- It doesn't support whitespace between '\' and the newline when looking if the '\' is used as a line continuation character.

This commit fixes both issues.

Differential Revision: https://reviews.llvm.org/D68436

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374127 91177308-0d34-0410-b5e6-96231b3b80d8