.clang-format

tests: allow Google's internal builds of clang-format to be used

These builds do not actually include any Google-specific formatting changes; the
only reason they don't include the LLVM version number is due to a decision to
elide the version number from *all* LLVM/clang projects.

For most builds of clang-format, even "unofficial" ones, the LLVM version will
be displayed; example:

```
clang-format version 14.0.0 (https://github.com/llvm/llvm-project.git 1830ec94ac022ae0b6d6876fc2251e6b91e5931e)
```

The Google-internal build looks like this:

```
clang-format version google3-trunk (1830ec94ac022ae0b6d6876fc2251e6b91e5931e)
```

Differential Revision: https://phab.mercurial-scm.org/D10538

BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
  - Regex:           '^<'
    Priority:        1
  - Regex:           '^"'
    Priority:        2