[NFCI] Refactor how KeywordStatus is calculated

The getKeywordStatus function is a horrible mess of inter-dependent 'if'
statements that depend significantly on the ORDER of the checks.  This
patch removes the dependency on order by checking each set-flag only
once.

It does this by looping through each of the set bits, and checks each
individual flag for its effect, then combines them at the end.

This might slow down startup performance slightly, as there are only a
few hundred keywords, and a vast majority will only get checked 1x
still.

This patch ALSO removes the KEYWORD_CONCEPTS flag, because it has since
become synonymous with C++20.

Differential Revision: https://reviews.llvm.org/D131007