[clang-format] Fix regression that breaks comments without a comment prefix

[krasimir]

Summary

Consider formatting the following code fragment with column limit 20:

{
  // line 1
  // line 2\
  // long long long line
}

Before this fix the output is:

{
  // line 1
  // line 2\
  // long long
  long line
}

This patch fixes a regression that breaks the last comment line without
adding the '//' prefix.

lib/Format/BreakableToken.cpp

for (const FormatToken *CurrentTok = &Tok;
     TokenText.split(Lines, "\n");
     Content.resize(Lines.size());
     ContentColumn.resize(Lines.size());
     OriginalContentColumn.resize(Lines.size());
     Tokens.resize(Lines.size());
     Prefix.resize(Lines.size());
     OriginalPrefix.resize(Lines.size());
     for (size_t i = FirstLineIndex, e = Lines.size(); i < e; ++i) {
-      StringRef IndentPrefix = getLineCommentIndentPrefix(Lines[i]);
+      // We need to trim the blanks in case this is not the first line in a
+      // multiline comment. Then the indent is included in Lines[i].
+      StringRef IndentPrefix =
+          getLineCommentIndentPrefix(Lines[i].ltrim(Blanks));
+      assert(IndentPrefix.startswith("//"));
       OriginalPrefix[i] = Prefix[i] = IndentPrefix;
       if (Lines[i].size() > Prefix[i].size() &&
           isAlphanumeric(Lines[i][Prefix[i].size()])) {
         if (Prefix[i] == "//")
           Prefix[i] = "// ";
         else if (Prefix[i] == "///")
           Prefix[i] = "/// ";
         else if (Prefix[i] == "//!")

unittests/Format/FormatTest.cpp

TEST_F(FormatTest, SplitsLongCxxComments) {
   EXPECT_EQ(
       "#define XXX // a b c d\n"
       "            // e f g h",
       format("#define XXX // a b c d e f g h", getLLVMStyleWithColumns(22)));
   EXPECT_EQ(
       "#define XXX // q w e r\n"
       "            // t y u i",
       format("#define XXX //q w e r t y u i", getLLVMStyleWithColumns(22)));
+  EXPECT_EQ("{\n"
+            "  //\n"
+            "  //\\\n"
+            "  // long 1 2 3 4\n"
+            "  // 5\n"
+            "}",
+            format("{\n"
+                   "  //\n"
+                   "  //\\\n"
+                   "  // long 1 2 3 4 5\n"
+                   "}",
+                   getLLVMStyleWithColumns(20)));
 }
 
 TEST_F(FormatTest, PreservesHangingIndentInCxxComments) {
   EXPECT_EQ("//     A comment\n"
             "//     that doesn't\n"
             "//     fit on one\n"
             "//     line",
             format("//     A comment that doesn't fit on one line",