Previously, if a request R evaluated itself N times, we would emit N
"circular reference" diagnostics. These add no value, so instead let's
cache the user-provided default value on the first circular evaluation.
This changes things slightly so that instead of returning an
llvm::Expected<Request::OutputType>, various evaluator methods take
a callback which can produce the default value.
The existing evaluateOrDefault() interface is unchanged, and a new
evaluateOrFatal() entry point replaces
llvm::cantFail(ctx.evaluator(...)).
Direct callers of the evaluator's operator() were updated to pass in
the callback. The benefit of the callback over evaluateOrDefault() is
that if the default value is expensive to constuct, like a dummy
generic signature, we will only construct it in the case where a
cycle actually happened, otherwise we just delete the callback.
(cherry picked from commit b8fcf1c709efa6cd28e1217bd0efe876f7c0d2b7)
Just as with the result cache, instead of a single DenseMap with
type-erased AnyRequest keys, we can use per-request maps for a
nice performance improvement.
This is based on an earlier patch by @hamishknight.
The idea is that instead of caching results in a single DenseMap
that maps AnyRequest to AnyValue, we instead define a separate
DenseMap for each request kind that directly uses the request as
the key, and the request value as the value.
This avoids type erasure and memory allocation overhead arising
from the use of AnyRequest and AnyValue. There are no remaining
usages of AnyValue, and the only usage of AnyRequest is now in
the reference dependency tracking, which can be refactored to use
a similar strategy of storing per-request maps as well.
