[clang-format] Add a default format style that can be used by users of `getStyle`

Summary:
Tools that reformat code often call `getStyle` to decide the format style
to use on a certain source file. In practice, "file" style is widely used. As a
result, many tools hardcode "file" when calling `getStyle`, which makes it hard
to control the default style in tools across a codebase when needed. This change
introduces a `DefaultFormatStyle` constant (default to "file" in upstream), which
can be modified downstream if wanted, so that all users/tools built from the same
source tree can have a consistent default format style.

This also adds an DefaultFallbackStyle that is recommended to be used by tools and can be modified downstream.

Reviewers: sammccall, djasper

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48492

llvm-svn: 335492