[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@335492
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