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
diff --git a/unittests/Format/FormatTest.cpp b/unittests/Format/FormatTest.cpp
index 29988d3..dc77c77 100644
--- a/unittests/Format/FormatTest.cpp
+++ b/unittests/Format/FormatTest.cpp
@@ -490,6 +490,7 @@
 //===----------------------------------------------------------------------===//
 
 TEST_F(FormatTest, UnderstandsSingleLineComments) {
+  verifyFormat("//* */");
   verifyFormat("// line 1\n"
                "// line 2\n"
                "void f() {}\n");