Support enum values in substitution XML

[sbomer]

Fixes #131601

• Allow ILLink enum substitutions to use numeric values parsed as the enum underlying type.
• Allow NativeAOT substitutions for int-backed enums to use member names.
• Add shared field and stubbed-method coverage for named and numeric values, including exact linked IL validation.

Tests:

• Mono.Linker.Tests.TestCases.All.SubstitutionsTests
• Mono.Linker.Tests.TestCases.All.Substitutions (NativeAOT)

Note

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Pull request overview

This PR makes enum-valued substitutions in ILLink.Substitutions.xml more consistent between ILLink (trimmer) and NativeAOT (ILCompiler) by accepting additional enum value forms, and adds regression coverage in both test suites.

Changes:

• ILLink: when substituting an enum value, fall back to parsing numeric strings using the enum’s underlying type.
• NativeAOT: for int-backed enums, allow substitution values to be specified by enum member name (resolved to the member’s constant).
• Tests: add a new substitutions test case and enable it in the NativeAOT trimming substitutions suite.

Show a summary per file

File	Description
src/tools/illink/test/Mono.Linker.Tests.Cases/Substitutions/EnumSubstitutions.xml	New substitution XML covering enum member names and numeric values for fields and stubbed methods.
src/tools/illink/test/Mono.Linker.Tests.Cases/Substitutions/EnumSubstitutions.cs	New linker test case asserting stubbed method IL for enum substitutions by name and number.
src/tools/illink/src/linker/Linker.Steps/ProcessLinkerXmlBase.cs	Extends enum substitution parsing to accept numeric values via underlying-type parsing.
src/coreclr/tools/aot/ILCompiler.Trimming.Tests/TestCases/TestSuites.cs	Enables the new test case in the NativeAOT trimming substitutions suite.
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/BodySubstitutionParser.cs	Adds enum-member-name resolution for int-backed enums when creating substitutions.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

Suppressed comments (1)

src/tools/illink/src/linker/Linker.Steps/ProcessLinkerXmlBase.cs:656

• The enum fallback currently only attempts numeric parsing when the underlying type is Int32. That doesn't match the stated goal of parsing numeric values using the enum's underlying type, and it means numeric substitutions will still fail for enums backed by byte/short/long/uint/etc (even though TryConvertValue already supports those primitive metadata types).

                        var underlyingType = typeDefinition.GetEnumUnderlyingType();
                        if (underlyingType.MetadataType == MetadataType.Int32)
                            return TryConvertValue(value, underlyingType, out result);

• Files reviewed: 5/5 changed files
• Comments generated: 0 new

[agocke]

LGTM, I think there are some ILTrim failures