Allow _clang-format as alternative to .clang-format config filename

Dotfiles are impractical on Windows. This makes clang-format search
for the style configuration file as '_clang-format' in addition to
the usual '.clang-format'. This is similar to how VIM searches for
'_vimrc' on Windows.

Differential Revision: http://llvm-reviews.chandlerc.com/D1629

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190413 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/clang-format/clang-format.py b/tools/clang-format/clang-format.py
index 4077b41..d8338ab 100644
--- a/tools/clang-format/clang-format.py
+++ b/tools/clang-format/clang-format.py
@@ -28,7 +28,8 @@
 
 # Change this to format according to other formatting styles. See the output of
 # 'clang-format --help' for a list of supported styles. The default looks for
-# a '.clang-format' file to indicate the style that should be used.
+# a '.clang-format' or '_clang-format' file to indicate the style that should be
+# used.
 style = 'file'
 
 # Get the current text.