main: keep the default IFormattable.ToString of IValueSetFactory out of ILCompiler's inlining (port of #315, dotnet/runtime#136073) #317

Port of #315 to main. That issue is closed: PR #316 landed on release/10.x as 3ed738e.

Problem

#291 kept the Native AOT trap (dotnet/runtime#136073) out of the 11 line by giving each value set family its
own IFormattable.ToString. That holds only while every family carries the override: a family added without
one compiles, passes every test on the SDK CI runs, and traps (exit 133, no output) only in a net11.0
Native AOT publish. IValueSetFactory<,> cannot be implemented outside this repository (FromTrusted is
internal abstract), so this is a guard for future families here, not a consumer-facing gap.

Change

[MethodImpl(MethodImplOptions.NoInlining)] on the default IFormattable.ToString of
IValueSetFactory<TSet, T>, the workaround hez2010 gave on dotnet/runtime#136073, plus a conventions test
asserting the flag by reflection. Measured on 10.x with SDK 11.0.100-rc.2.26504.105: the 10.0.0 state of the
sets exits 133 without it and runs to the end with it (details in #315). No public surface or answer changes.

Done when

• The change is on main, a cherry-pick (-x) of the 10.x commit, with the conventions test passing.
• When NativeAOT (.NET 11 RC2, osx-arm64): SIGTRAP in the instantiating stub of a default interface IFormattable.ToString on a shared-generic interface dotnet/runtime#136073 is fixed, the overrides and the attribute are revisited together; this issue
  does not remove either.