[clang-format] Fix regression about adding leading whitespace to the content of line comments

[krasimir]

Summary

The reflower didn't measure precisely the line column of a line in the middle of
a line comment section that has a prefix that needs to be adapted.

source:

/// a
//b

format before:

/// a
 //b

format after:

/// a
// b

[krasimir]

• Removed newline

[krasimir]

• Updated obsolete comments

[djasper]

Looks good.

lib/Format/BreakableToken.cpp

if (SplitBefore.first != StringRef::npos) {
                                            /*PreviousPostfix=*/"",
                                            /*CurrentPrefix=*/ReflowPrefix,
                                            /*InPPDirective=*/false,
                                            /*Newlines=*/0,
                                            /*Spaces=*/0);
     } else {
       // This is the first line for the current token, but no reflow with the
       // previous token is necessary. However, we still may need to adjust the
-      // start column.
+      // start column. Note that ContentColumn[LineIndex] is the expected
+      // content column after a possible update to the prefix, hence the prefix
+      // length change is included.
       unsigned LineColumn =
           ContentColumn[LineIndex] -
-          (Content[LineIndex].data() - Lines[LineIndex].data());
+          (Content[LineIndex].data() - Lines[LineIndex].data()) +
+          (OriginalPrefix[LineIndex].size() - Prefix[LineIndex].size());
       if (tokenAt(LineIndex).OriginalColumn != LineColumn) {
         Whitespaces.replaceWhitespace(*Tokens[LineIndex],
                                       /*Newlines=*/1,
                                       /*Spaces=*/LineColumn,
                                       /*StartOfTokenColumn=*/LineColumn,
                                       /*InPPDirective=*/false);
       } else {
         // The whitespace preceding the first line of this token does not need
         // to be touched.
         Whitespaces.addUntouchableToken(tokenAt(LineIndex),
                                         /*InPPDirective=*/false);
       }
     }
-  } else if (OriginalPrefix[LineIndex] != Prefix[LineIndex]) {
+  }
+  if (OriginalPrefix[LineIndex] != Prefix[LineIndex]) {
     // This is not the first line of the token. Adjust the prefix if necessary.
 
     // Take care of the space possibly introduced after a decoration.
     assert(Prefix[LineIndex] == (OriginalPrefix[LineIndex] + " ").str() &&
            "Expecting a block comment decoration to differ from original by "
            "at most a space");
     Whitespaces.replaceWhitespaceInToken(
         tokenAt(LineIndex), OriginalPrefix[LineIndex].size(), 0, "", "",

unittests/Format/FormatTest.cpp

EXPECT_EQ("//! Add leading\n"
             "//! whitespace",
             format("//!Add leading whitespace", getLLVMStyleWithColumns(20)));
   EXPECT_EQ("// whitespace", format("//whitespace", getLLVMStyle()));
   EXPECT_EQ("// Even if it makes the line exceed the column\n"
             "// limit",
             format("//Even if it makes the line exceed the column limit",
                    getLLVMStyleWithColumns(51)));
   EXPECT_EQ("//--But not here", format("//--But not here", getLLVMStyle()));
+  EXPECT_EQ("/// line 1\n"
+            "// add leading whitespace",
+            format("/// line 1\n"
+                   "//add leading whitespace",
+                   getLLVMStyleWithColumns(30)));
+  EXPECT_EQ("/// line 1\n"
+            "/// line 2\n"
+            "//! line 3\n"
+            "//! line 4\n"
+            "//! line 5\n"
+            "// line 6\n"
+            "// line 7",
+            format("///line 1\n"
+                   "///line 2\n"
+                   "//! line 3\n"
+                   "//!line 4\n"
+                   "//!line 5\n"
+                   "// line 6\n"
+                   "//line 7", getLLVMStyleWithColumns(20)));
 
   EXPECT_EQ("// aa bb cc dd",
             format("// aa bb             cc dd                   ",
                    getLLVMStyleWithColumns(15)));
 
   EXPECT_EQ("// A comment before\n"
             "// a macro\n"
             "// definition\n"