This enables sharing the logic between standalone clangd and embedders
of it. The new approach should be same performance-wise, as it is only called
once per addDocument call.
This patch also introduces a blacklisting code path for disabling crashy or
high-noise tests, until we figure out a way to make them work with clangd-setup.
The biggest difference is the way we make use of preambles, hence those checks
can't see directives coming from the preamble section of the file. The second
thing is the fact that code might-not be compiling while clangd is trying to
build an AST, hence some checks might choke on those incomplete ASTs.