[clang-format] Fix reflow in block comment lines with leading whitespace.

Summary:
The reflower was not taking into account the additional  leading whitespace in block comment lines.

source:
```
{
/*
 * long long long long
 *   long
 * long long long long
 */
}
```

format (with column limit 20) before:
```
{
  /*
   * long long long
   * long long long long
   * long long
   */
}
```
format after:
```
{
  /*
   * long long long
   * long long long
   * long long long
   */
}
```

Reviewers: djasper, klimek

Reviewed By: djasper

Subscribers: cfe-commits, sammccall, klimek

Differential Revision: https://reviews.llvm.org/D29326

llvm-svn: 293633
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index fb77006..1097514 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -1960,6 +1960,23 @@
                    " * longg */",
                    getLLVMStyleWithColumns(20)));
 
+  // Reflow lines with leading whitespace.
+  EXPECT_EQ("{\n"
+            "  /*\n"
+            "   * long long long\n"
+            "   * long long long\n"
+            "   * long long long\n"
+            "   */\n"
+            "}",
+            format("{\n"
+                   "/*\n"
+                   " * long long long long\n"
+                   " *   long\n"
+                   " * long long long long\n"
+                   " */\n"
+                   "}",
+                   getLLVMStyleWithColumns(20)));
+
   // Break single line block comments that are first in the line with ' *'
   // decoration.
   EXPECT_EQ("/* long long long\n"