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

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
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: sammccall, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29329

llvm-svn: 293641
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 1097514..b316350 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -1371,6 +1371,25 @@
             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                   ",