Defer unused CoreLib cache initialization

[tannergooding]

Moves the ICU-only CompareInfo search values and the EncodingTable name cache behind nested static holders. This preserves one-time initialization when those paths are used while allowing NativeAOT to trim them from applications that only need unrelated static state.

A locally built Release NativeAOT win-x64 Hello World with InvariantGlobalization=true measured:

Build	Executable	Delta
main	993,280 bytes	—
CompareInfo holder	971,264 bytes	-22,016 bytes
Both holders	968,704 bytes	-24,576 bytes

The cumulative raw section changes are .text -12,800 bytes, .rdata -9,728 bytes, .data -1,024 bytes, .pdata -512 bytes, and .reloc -512 bytes. The final dependency graph no longer contains SearchValues.Create, EncodingTable::.cctor, or the ConcurrentDictionary<string, int> name-cache construction.

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Pull request overview

This PR defers initialization of two rarely-needed CoreLib caches by moving them behind nested static holder types, allowing NativeAOT trimming to omit the cache initialization (and related dependencies) when those paths aren’t used.

Changes:

• Move the EncodingTable name→codepage ConcurrentDictionary into a nested static holder to avoid initializing it unless GetCodePageFromName is called.
• Move the ICU-only CompareInfoSearchValues<char> initialization into a nested static holder to avoid creating it unless the ASCII fast-path is exercised.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Text/EncodingTable.cs	Defers ConcurrentDictionary<string,int> construction until GetCodePageFromName is invoked.
src/libraries/System.Private.CoreLib/src/System/Globalization/CompareInfo.Icu.cs	Defers ICU ASCII SearchValues<char> creation until the ordinal helper methods need it.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

[tannergooding]

A sibling to #132550. Just did some local analysis for obvious static constructor roots that looked unnecessary

-- CC. @MichalStrehovsky, @jkotas, @agocke this is a trivial improvement that's likely worth taking for .NET 11/12, but it also feels like a more general issue and is a really common pattern in how we and the broader ecosystem write code. This is likely worth explicitly handling in the linker, either via some explicit trimming support or a way (maybe even an attribute) to indicate we support it being "outlined" by the tooling into such helper shapes.

We shouldn't have to explicitly think that every unique static readonly field may require its own nested holder class. At worst this should be some analyzer that flags it for NAOT enabled libraries with a fixer if we believe handling it in the linker is not feasible.

[Copilot]

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Lite

[tannergooding]

Updated everything and savings measured locally are still the same.