Use internal metadata interfaces in CoreCLR

[davidwrighton]

Summary

• migrate Reflection.Emit metadata operations from the legacy public import and emit interfaces to IMDInternalImport and IMDInternalEmit
• retain internal metadata interfaces directly in dynamic PEAssembly instances and keep public VM bridges only in profiler-enabled builds
• omit the legacy public import, emit, tables, and metadata-info interfaces from runtime RegMeta builds without profiler support while retaining them for profiler, DAC, DBI, and portable-PDB targets
• omit metadata persistence, dynamic metadata snapshots, pre-save processing, and stream serialization from WebAssembly runtime metadata while preserving writable Reflection.Emit metadata
• reduce MDTOKENMAP to the sparse source-token movement map needed by desktop persistence and remove unused metadata lookup helpers
• return E_NOTIMPL from the CoreCLR copies of internal APIs used only by ILAsm or ILDasm while retaining their implementations in the portable-PDB tool flavor
• compile the no-op symbol-reader importer only when FEATURE_ISYM_READER is enabled

WebAssembly size

Measured with ./build.sh -os browser -c Release -subset clr+libs against a clean worktree at bae5a8d2f1913c45bd474fdbecb17d572e235a4c.

Artifact	Baseline	Current	Change
dotnet.native.wasm	4,714,654 bytes	4,612,862 bytes	-101,792 bytes (-2.16%)
dotnet.native.wasm (gzip -9)	1,784,275 bytes	1,749,185 bytes	-35,090 bytes (-1.97%)
corerun.wasm	4,542,104 bytes	4,440,392 bytes	-101,712 bytes (-2.24%)
corerun.wasm (gzip -9)	1,742,265 bytes	1,706,092 bytes	-36,173 bytes (-2.08%)

The final Emscripten symbol map contains no public IMetaDataTables, IMetaDataTables2, or IMetaDataInfo implementation; metadata persistence implementation; removed MDTOKENMAP modes or lookup helpers; or full CoreCLR implementation of the two tool-only APIs. Required interface slots remain as nine-byte E_NOTIMPL stubs.

Testing

• ./build.sh clr+libs+host — passed with 0 warnings and 0 errors
• ./build.sh -os browser -c Release -subset clr+libs — passed with 0 warnings and 0 errors
• ./dotnet.sh build src/libraries/System.Reflection.Emit/tests/System.Reflection.Emit.Tests.csproj -t:Test -p:TargetArchitecture=x64 -p:Configuration=Debug — 2,640 passed, 0 failed, 0 skipped

Note

This pull request description was generated with GitHub Copilot.

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[davidwrighton]

/ba-g failing tests are known, and fixed while this PR was open, so the failures are expected.