Make clang-format understand more line comments.

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:
//****************

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177720 91177308-0d34-0410-b5e6-96231b3b80d8