Add AccessModifierOffset to clang-format style

Summary:
As we override the indent option of the LLVM style, we need to override the access modifier
offset as well. Otherwise, classes will be formatted like such
class A
{
  public:
    int foo;
};
which is not used anywhere in LLDB. This option makes clang-format style more similar to LLDB and
brings it closer to the original intention of LLVM style, which was to not indent access
modifiers.

Reviewers: zturner, tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D15562

llvm-svn: 255882