clang-format: Add ability to align assignment operators.

In Objective-C some style guides use a style where assignment operators are
aligned, in an effort to increase code readability. This patch adds an option
to the format library which allows this functionality. It is disabled by
default for all the included styles, so it must be explicitly enabled.

The option will change code such as:
  - (void)method {
      NSNumber *one = @1;
      NSNumber *twentyFive = @25;
  }

to:
  - (void)method {
      NSNumber *one        = @1;
      NSNumber *twentyFive = @25;
  }

Patch by Matt Oakes. Thank you!

Accidentally reformatted all the tests...

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236100 91177308-0d34-0410-b5e6-96231b3b80d8