Fix #3754: omit async stepping info for runtime-async methods

[siegfriedpammer]

Fixes #3754.

Problem

Generating a portable PDB for an assembly containing .NET 11 runtime-async methods threw a NullReferenceException instead of producing a PDB:

System.NullReferenceException
   at ICSharpCode.Decompiler.DebugInfo.AsyncDebugInfo.BuildBlob(MethodDefinitionHandle moveNext)
   at ICSharpCode.Decompiler.DebugInfo.PortablePdbWriter.WritePdb(...)

Root cause

ILFunction.IsAsync is derived from the method signature, so runtime-async methods (MethodImplAsync bit, no MoveNext state machine) report IsAsync without AsyncAwaitDecompiler ever populating AsyncDebugInfo. Its Awaits then stays an uninitialized ImmutableArray<Await>, and PortablePdbWriter threw building the MethodSteppingInformation blob from that default struct.

Fix

Guard emission with !function.AsyncDebugInfo.Awaits.IsDefault. Runtime-async methods have no yield/resume offsets to record, so the stepping blob is omitted, matching the C# compiler (which emits no stepping information for them either). A genuinely zero-await classic state machine keeps an initialized empty Awaits and is unaffected (IsDefault, not IsDefaultOrEmpty).

Test

Adds a PdbGen/RuntimeAsync.cs fixture and a PdbGenerationTestRunner.RuntimeAsync test that compiles a runtime-async method in-proc (new runtimeAsync flag on Tester.CompileCSharpWithPdb, mirroring the out-of-process /features:runtime-async=on), runs PortablePdbWriter, and asserts it does not throw and emits no MethodSteppingInformation. The test fails with the NRE before the fix and passes after; the full PdbGen fixture stays green.

Verified end-to-end with ilspycmd -genpdb on a minimal runtime-async assembly (valid PDB, no stepping info) and on the issue's System.Net.Requests.dll (past the NRE).

🤖 Generated with Claude Code