[RuntimeAsync] Move runtime async to the main repo

[VSadov]

Do not review just yet. May need some work.

A list of things to address. Some of these may be more relevant to address separately later.

Required before merge:

• Resolve build issues.
• Resolve test failures in existing tests
• Asses the performance impact from the presence of the feature.
• Make sure no new GC Stress failures.
• Restore CODEOWNERS
• Determine what to do about CompatibilitySuppressions
• Decide whether we want a feature ifdef and merge this disabled, or whether we can do the work upfront to merge it enabled
• Ensure new files have license headers
• (JIT) Add function headers on new functions
• Make sure crossgen2/ilc do not blow up on async2 methods
• Undo NativeCodeVersion/NativeCodeVersionNode/PrepareCodeConfig slot changes (not used in the end)
• Undo codeman.cpp white space change
• Figure out if eetwain.cpp change should be undone
• Figure out if gcinfodecoder.cpp change should be undone
• Remove auto in ClassifyAsyncMethodSignatureCore
• Add proper contracts and declarations for jitinterface.cpp methods for building resumption stubs
• Delete restoreregs_for_runtimesuspension.h
• Address stack underflow changes in stubgen.cpp
• Remove added ILCodeStream::EmitUNBOX if it is unused
• Undo QCallHandles.cs change
• Decide whether meaningful names of getAsyncResumptionStub should be debug-only
• Decide if we should keep performance measurements in design doc
• Find and replace all instances of "async2" with "runtime async", or just "async" when it is clear that it is runtime async

Clean ups that are potentially not necessary for this PR (but some of these may be nice-to-have before review):

• (VM) Unify new OSR overload with existing overload
• (JIT/VM) Decide whether to remove new CORINFO_HELP_THROWEXACT in favor of using ExceptionDispatchInfo
• (JIT) Split up async2 transformation into smaller documented parts
• (JIT) Replace GenTreeCall::gtIsAsyncCall with a flag or calling convention change
• (JIT) Best-effort LA64/RISCV64 support
• (JIT/VM) Remove the "StructMethodThatOperatesOnCopy" mechanism (in favor of Roslyn-created copy)
• (JIT) Remove unnecessary intrinsic recognitions from special await helpers and get_RuntimeAsyncViaJitGeneratedStateMachines
• (VM) Rewrite MethodTableNative::GetLoaderAllocatorHandle in managed code
• (VM) Decide whether async thunks should be allowed to tier
• (VM) Optimize MethodTable::GetParallelMethodDesc

[jkotas]

hide async2 variants from reflection

Find and replace all instances of "async2" with "runtime async", or just "async" when it is clear that it is runtime async?

[VSadov]

Find and replace all instances of "async2" with "runtime async", or just "async" when it is clear that it is runtime async?

I am considering that. The 2 in async2 is not very meaningful at runtime level, since there is only one kind of async.
We have cleaned JIT from 2, somewhat, already.

It is just a mostly mechanical change that is further on the list of things to do.

[VSadov]

I will close this now since we have #114675 and #114861