[Format] Rename .clang_format to .clang-format to enable it

[xumingkuan]

I think .clang-format should not be in .gitignore so we can configure the same code format. See .clang_format → .clang-format for the nominal changes.

Actual code format change in this commit:

• Access modifiers: Before: not specified After:

  class A {
  public:
  int a;
  };

  instead of

  class A {
  public:
  int a;
  };

Some suggestions in the future:

• IncludeBlocks: Preserve -> Regroup
• SpacesBeforeTrailingComments: 1 -> 2