Fix test containing backslash and newline separated by space

This was silently accepted by clang without warning due to a lexer bug.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197330 91177308-0d34-0410-b5e6-96231b3b80d8
Lexer: Issue -Wbackslash-newline-escape for line comments

The warning for backslash and newline separated by whitespace was missed in
this code path.

backslash<whitespace><newline> is handled differently from compiler to compiler
so it's important to warn consistently where there's ambiguity.

Matches similar handling of block comments and non-comment lines.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197331 91177308-0d34-0410-b5e6-96231b3b80d8