Remove cxxopt flags when linting C sources.

[agoessling]

Previously cxxopts were used even if the source was a C file. Now cxxopts and copts are passed correctly to clang-tidy depending on the file type.

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Changes are visible to end-users: no

Test plan

• Manual testing; please provide instructions so we can reproduce:

Include a C++ only flag in .bazelrc e.g. build --cxxopt="-std=c++17" and lint a C target.

[alexeagle]

thanks!