Analyzers: Build MUD0002 component descriptors only on cache misses

[danielchalmers]

First of four stacked PRs that speed up the MUD0002 analyzer added in #13831 and #13841.

_componentDescriptors.GetOrAdd(type, ComponentDescriptor.GetComponentDescriptor(...)) built the descriptor before the lookup, so every OpenComponent walked the component's members and scanned the migration catalog, even when the descriptor was already cached.
This passes a cached Func to GetOrAdd instead, so a cache hit does no work and allocates nothing.
netstandard2.0 has no GetOrAdd<TArg> overload, hence the stored delegate.
Concurrent misses can still build a descriptor twice; GetOrAdd keeps the first, as before.

Scenario	Before	After	Paired change, median [IQR]
MudBlazor.Docs build, analyzer time	649 ms	414 ms	−293 ms [−359, −132]
MudBlazor.Docs build, analyzer allocations	103.5 MB	69.9 MB	−32.7 MB [−34.8, −30.0]
MudBlazor.Docs editor analysis, analyzer time	58.1 ms	51.6 ms	−6.1 ms [−10.6, −4.2]
MudBlazor build, analyzer time	51.0 ms	44.7 ms	−5.0 ms [−9.9, −2.5]
20 files × 200 components over 16 types	164 ms, 67.4 MB	7.4 ms, 6.2 MB	−153 ms, −61.1 MB

Descriptors built while analyzing MudBlazor.Docs drop from 8,176 (one per lookup) to 1,755.
Most of the remaining 1,755 come from inferred generic components, which the third PR in this stack addresses.

How this was measured

• Inputs are already-built compilations: MudBlazor.Docs (1,189 trees including 1,037 Razor pages) and MudBlazor net10.0 (893 trees), recreated from their own compiler command lines with source-generator output emitted, plus synthetic Razor-shaped files.
• Each run creates a fresh CompilationWithAnalyzers, so no cached results are reused.
• Each variant's analyzer DLL is loaded side by side; runs alternate between variants and each change is paired against the previous PR within the same round.
• Serial analyzer driver, 20 runs after 3 warmups (6 runs for whole-project builds).
• Analyzer time is Roslyn's per-analyzer execution time.
• Allocations are GC.GetTotalAllocatedBytes(precise: true) across all threads, minus a no-op analyzer that registers the same actions.
• "Editor analysis" is semantic diagnostics for MudBlazor.Docs' 12 largest generated Razor documents, one document at a time.
• Descriptor counts come from temporary counters that are not part of this PR.
• Windows 11, .NET 10.0.12, Roslyn 5.9 host, analyzer built against its Roslyn 4.7.0 floor, measured on 13e4c48.