[clang-tools-extra] feat: highlight for import, export keyword of CXX Module (PR #204511)

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Decodetalkers wrote:

I do not know how to add the unit test...

seems the unit test code should be

$keyword[[default]], but clearly This snippet cannot be tokenlized. but other thing, like template, void, they also has its highlightToken, but they do not need something like $Primtive[void], I am confused

https://github.com/llvm/llvm-project/pull/204511