[clang-tidy] Add allow list to `google-build-using-namespace`

[JungPhilipp]

The clang-tidy check google-build-using-namespace warns on almost all using namespace x directives.

There seem to exceptions to this rule, one of which are already part of the check. E.g., for (std) literal namespaces. (clang-tools-extra/clang-tidy/google/UsingNamespaceDirectiveCheck.cpp:31).

Many libraries/projects have their own literal namespaces or standard libraries where this exception does not work.
They currently either cannot use the check at all or have to litter the code with //NOLINT comments.

using namespace ::testing from gtest/gmock also seems to be used frequently.

Suggested solution:
Add an allow-list for which namespaces warnings are omitted.
Similar to

• StringLikeClasses of performance-faster-string-find
• IgnoredFilesList of misc-header-include-cycle
• Various options of modernize-use-emplace

I can provide a PR, just wanted to get feedback before putting in the work.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Philipp Jung (JungPhilipp)

The clang-tidy check `google-build-using-namespace` warns on almost all `using namespace x` directives.

There seem to exceptions to this rule, one of which are already part of the check. E.g., for (std) literal namespaces. (clang-tools-extra/clang-tidy/google/UsingNamespaceDirectiveCheck.cpp:31).

Many libraries/projects have their own literal namespaces or standard libraries where this exception does not work.
They currently either cannot use the check at all or have to litter the code with //NOLINT comments.

using namespace ::testing from gtest/gmock also seems to be used frequently.

Suggested solution:
Add an allow-list for which namespaces warnings are omitted.
Similar to

• StringLikeClasses of performance-faster-string-find
• IgnoredFilesList of misc-header-include-cycle
• Various options of modernize-use-emplace

I can provide a PR, just wanted to get feedback before putting in the work.

[vbvictor]

Seems reasonable, I don't see any objections. Just put std as the default value of this new option.