[clangd] More precisely enable clang warnings through ClangTidy options

clang-tidy's behavior is to add the -W flags, and then map all clang diagnostics
to "clang-diagnostic-foo" pseudo-check-names, then use Checks to filter those.

Previous to this patch, we were handling -W flags but not filtering the
diagnostics, assuming both sets of information encoded the same thing.

However this intersection is nontrivial when diagnostic group hierarchy is
involved. e.g. -Wunused + clang-diagnostic-unused-function should not enable
unused label warnings.

This patch more closely emulates clang-tidy's behavior, while not going to
the extreme of generating tidy check names for all clang diagnostics and
filtering them with regexes.

Differential Revision: https://reviews.llvm.org/D124679