Make clang-format understand more line comments.

Apparently one needs to set LangOptions.LineComment.

Before "//* */" got reformatted to "/ /* */" as the lexer was returning
the token sequence (slash, comment). This could also lead to weird other
stuff, e.g. for people that like to using comments like:
//****************
Fix DeclRefExpr::getFoundDecl() for usages by reference.