Improved default style tests.

Renamed DontModify enum to Leave.

Rebased changes on master.

Diff updated. Previous diff was generated after rebase, and Phabricator change preview did not show any unrelated changes, so I thought that everything is fine.

Switched EmptyLineBeforeAccessModifierStyle option from bool to enum.
EmptyLineBeforeAccessModifierStyle option can now have one of four values: Never, DontModify, LogicalBlock, Always.
Never removes all empty lines before access modifiers.
DontModify keeps existing empty lines.
LogicalBlock adds empty line only when access modifier starts a new logical block (current clang-format behavior).
Always adds empty line before all access modifiers except when access modifier is at the start of class/struct definition.

When access modifier is in the same line with previous tokens, UnwrappedLineFormatter::formatFirstToken is called with RootToken.NewlinesBefore == 0, but empty line is only inserted if RootToken.NewlinesBefore == 1. The following change fixes this and passes all tests except the ones with comment:

   if (PreviousLine && RootToken.isAccessSpecifier()) {
     if (Style.EmptyLineBeforeAccessModifier &&
         PreviousLine->Last->isOneOf(tok::semi, tok::r_brace) &&
-        RootToken.NewlinesBefore == 1)
-      ++Newlines;
+        RootToken.NewlinesBefore <= 1)
+      Newlines = 2;
     else if (!Style.EmptyLineBeforeAccessModifier &&
              RootToken.NewlinesBefore > 1)
       Newlines = 1;
   }

After some updating, rebuilding and searching for differences in Objective-C++ formatting, I managed to find where the problem with these failing tests is. In _verifyFormat function C++ code formatting is tested for stability like this:

EXPECT_EQ(Expected.str(), format(Expected, Style));
EXPECT_EQ(Expected.str(), format(Code, Style));

, but Objective-C++ test, in the same function, looks like this:

EXPECT_EQ(Expected.str(), format(test::messUp(Code), ObjCStyle));

test::messUp function removes all newlines and indentation, so test code:

struct foo {
  int i;

Added missing full stop.
Executed clang/doc/tools/dump_style.py to update ClangFormatStyleOptions.rst.

Switched to verifyFormat in most of the tests.
Rerun clang-format.

Option renamed to EmptyLineBeforeAccessModifier.
Placed new configuration member in correct place alphabetically.
Last token in previous line is no longer checked when EmptyLineBeforeAccessModifier is false.
Executed clang/doc/tools/dump_style.py to update ClangFormatsStyleOptions.rst.
Unit tests added to clang/unittests/Format/FormatTests.cpp.