Summary of the changes (Less than 80 chars)

• Improved performance of XML docs inference by approximately 3.5, while using only about a third of the previous memory allocation.
• Ensured thread safety and prevented the modification of (cached) values that are retrieved from the XmlDocumentationFileResolver

Related to #8775 (in this specific format)

As discussed in #8775, the current implementation was quite slow.
I intentionally did not introduce any big conceptual changes (yet) and focused only on optimizing the current implementation.

According to the benchmarks below, the general performance has improved by a factor between 2 and 22 (without the inheritdoc-cases, see below for them) - on average around ×ばつ faster - while memory usage has been reduced significantly to around a third.

For inheritdoc-cases, the situation is a bit different: I discovered that the current implementation mutates the XElement instances retrieved from the XmlDocumentationFileResolver, effectively modifying its internal cache.

Here :

and here:

This behavior is both undesired (at least I think so) and not thread-safe.

To fix this, I changed the logic to create a copy of the XElement when resolving inheritdoc elements.
This ensures that the cache remains intact and that the implementation is thread-safe.
While this introduces a small performance regression for two out of six inheritdoc cases, I think it is justified since the previous implementation was incorrect and should not be considered a valid baseline: It only replaced the inheritdoc element once within the cached(!) XDocument, meaning that all subsequent calls for the same(!!!) member (which happens all the time in the benchmark) did not need to replace it again and threfore resulting in slightly better, but ultimately incomparable, performance characteristics. Of course, the current behavior could indeed be beneficial at runtime as well, f. e. an interface with many inheritors, but it is not that common as the benchmark indicates.

Overall, even for the inheritdoc-cases, the performance is still improved or comparable to before, and the memory usage is significantly lower:

Note: The PR currently includes the benchmark code so that one can reproduce the results.
I would to remove these benchmarks after the review and applying any suggested change.

Note further: This is a breaking change since I reworked XmlDocumentationFileResolver to become XmlDocumentationResolver.
This change is not strictly neccessary, but IIRC it was a 10-20% speed boost - I will reevaluate the concret impact on request if the change seems unlikely to be acceptable.

Full benchmarks:

BenchmarkDotNet v0.15.4, Windows 11 (10.0.26100.6584/24H2/2024Update/HudsonValley)
AMD Ryzen 7 7840HS with Radeon 780M Graphics 3.80GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK 9.0.203
 [Host] : .NET 9.0.8 (9.0.8, 9.0.825.36511), X64 RyuJIT x86-64-v4
 ShortRun : .NET 9.0.8 (9.0.8, 9.0.825.36511), X64 RyuJIT x86-64-v4
Job=ShortRun IterationCount=3 LaunchCount=1 
WarmupCount=3