[clang-format] don't break up #-style comment sections

[krasimir]

Summary

Follow-up from https://github.com/llvm/llvm-project/commit/36d13d3f8adb3d1a6bae71370afa23d11a94dc78; https://reviews.llvm.org/D121451.

Restore the old behavior in situations where we use # as comments and long strings of #'s for comment sections.

[MyDeveloperDay]

I hit something like this today with some code which was

//#undef

I'm not sure I think that

// #undef

or

//# undef

is what I want either

clang/lib/Format/BreakableToken.cpp

for (size_t i = FirstLineIndex, e = Lines.size(); i < e; ++i) {
         assert(Lines[i].size() > IndentPrefix.size());
         const char FirstCommentChar = Lines[i][IndentPrefix.size()];
         const unsigned FirstCharByteSize =
             encoding::getCodePointNumBytes(FirstCommentChar, Encoding);
         if (encoding::columnWidth(
                 Lines[i].substr(IndentPrefix.size(), FirstCharByteSize),
                 Encoding) != 1)
           return false;
-        if (FirstCommentChar == '#')
+        // In C-like comments, add a space before #. For example this is useful
+        // to preserve the relative indentation when commenting out code with
+        // #includes.
+        //
+        // In languages using # as the comment leader such as proto, don't
+        // add a space to support patterns like:
+        // #########
+        // # section
+        // #########
+        if (FirstCommentChar == '#' && !TokenText.startswith("#"))
           return false;
         return FirstCommentChar == '\\' || isPunctuation(FirstCommentChar) ||
                isHorizontalWhitespace(FirstCommentChar);
       };
 
       // On the first line of the comment section we calculate how many spaces
       // are to be added or removed, all lines after that just get only the
       // change and we will not look at the maximum anymore. Additionally to the

clang/unittests/Format/FormatTestTextProto.cpp

EXPECT_EQ("aaa: 100\n"
                    "## another double-hash comment.\n"
                    "###a triple-hash comment\n"
                    "cc: 200\n"
                    "### another triple-hash comment\n"
                    "####a quadriple-hash comment\n"
                    "dd: 100\n"
                    "#### another quadriple-hash comment\n",
                    Style));
+
+  // Ensure we support a common pattern for naming sections.
+  EXPECT_EQ("##############\n"
+            "# section name\n"
+            "##############",
+            format("##############\n"
+                   "# section name\n"
+                   "##############",
+                   Style));
+
+  EXPECT_EQ("///////////////\n"
+            "// section name\n"
+            "///////////////",
+            format("///////////////\n"
+                   "// section name\n"
+                   "///////////////",
+                   Style));
 }
 
 TEST_F(FormatTestTextProto, FormatsExtensions) {
   verifyFormat("[type] { key: value }");
   verifyFormat("[type] {\n"
                "  keyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy: value\n"
                "}");
   verifyFormat("[type.type] { key: value }");