Add external includes to clang-tidy invocation

[michael-ahn]

• This uses bazel's external_includes to account for external dependencies' headers (e.g. @my_library in cc_library.deps)
  • Current workaround is to manually add paths via bazel flags, e.g. build:clang-tidy --copt=-isystem./external/my_library/include; with this change this is no longer needed

[eneev-a]

This would be good to get in:

https://bazel.build/rules/lib/builtins/CompilationContext#external_includes says that these external_includes usually included with angle brackets:

depset CompilationContext.external_includes Returns the set of search paths (as strings) for external header files referenced by angle bracket. Usually passed with -isystem.

[dotnwat]

+1

[erenon]

Thanks! Merged.