Fix test containing backslash and newline separated by space

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