…g indirection

For methods versioned with vtable slot backpatching (virtual/interface
methods), the call counting infrastructure previously allocated separate
forwarder stub precodes to serve as stable indirection points between
vtable slots and deletable call counting stubs. This added memory
overhead and complexity for tracking, resetting, and trimming these
per-method forwarder stubs.

This change reuses the method's own temporary entry point (precode) as
the stable indirection during call counting. Instead of allocating a
forwarder stub and backpatching vtable slots to it, vtable slots are
reset to point to the method's precode, and the precode target is
redirected to the call counting stub via SetTargetInterlocked. This
preserves the safety property that vtable slots never point directly to
deletable call counting stubs, while eliminating the separate forwarder
stub allocation entirely.

Components updated:

- CallCountingManager::SetCodeEntryPoint: For backpatchable methods,
  replaced forwarder stub creation with ResetCodeEntryPoint (to ensure
  vtable slots point to the precode) followed by SetTargetInterlocked
  on the method's precode. Non-final-tier entry point transitions
  (threshold reached, pending completion) also update the precode
  target instead of backpatching vtable slots.

- CallCountingManager::CompleteCallCounting: For backpatchable methods,
  updates the precode target to native code or resets it to prestub,
  rather than calling SetCodeEntryPoint/ResetCodeEntryPoint which would
  backpatch vtable slots.

- CallCountingManager::StopAllCallCounting: For backpatchable methods,
  resets the precode target via ResetTargetInterlocked instead of
  calling ResetCodeEntryPoint. Removed the forwarder stub reset loop.

- CallCountingManager::DeleteAllCallCountingStubs: Removed forwarder
  stub lookup and removal from the per-method cleanup loop.

- CallCountingManager::TrimCollections: Removed forwarder stub hash
  table trimming.

- callcounting.h: Removed MethodDescForwarderStubHashTraits,
  MethodDescForwarderStubHash, and the m_methodDescForwarderStubHash
  member. Updated header documentation to reflect the new design.

When redirecting the method's temporary entry point (precode) to a call
counting stub, the previous code called ResetCodeEntryPoint() which set
GetMethodEntryPoint() to the temporary entry point value. This broke
DoBackpatch() slot recording because DoBackpatch() treats
GetMethodEntryPoint() == GetTemporaryEntryPoint() as 'method not yet
published' and skips slot discovery and recording.

Replace ResetCodeEntryPoint() with BackpatchToResetEntryPointSlots()
followed by SetMethodEntryPoint(codeEntryPoint). This resets vtable slots
to the temporary entry point (so calls flow through the precode) while
keeping GetMethodEntryPoint() at the native code entry point. After call
counting stubs are deleted and the precode reverts to prestub, DoBackpatch()
will correctly record new vtable slots because GetMethodEntryPoint() differs
from GetTemporaryEntryPoint().

Updated documentation in prestub.cpp, callcounting.cpp, and callcounting.h
to describe the precode redirect lifecycle and the bounded window during
call counting where new vtable slots may not be recorded.

For backpatchable methods (virtual/interface with vtable slot backpatching),
the temporary entry point's precode target must only ever be:
  1. The prestub (default, and when call counting is not active)
  2. A call counting stub (during active call counting only)

Previously, several code paths set the precode target to native code when
call counting ended (CompleteCallCounting, SetCodeEntryPoint with completed
stages, call count threshold hit). This bypassed DoBackpatch() slot
recording: new vtable slots would jump directly to native code without
being discovered and recorded, so future code version changes (e.g. ReJIT)
could not update those slots.

Change all precode updates that previously used SetTargetInterlocked(native)
to use ResetTargetInterlocked() (which resets to prestub). This ensures:
- SetCodeEntryPoint() handles recorded slots via BackpatchEntryPointSlots()
- The precode reverts to prestub so new slots flow through DoBackpatch()
- DeleteAllCallCountingStubs includes a safety-net reset for Stage::Complete

Updated components:
- callcounting.cpp: All SetCodeEntryPoint paths, CompleteCallCounting,
  StopAllCallCounting, and DeleteAllCallCountingStubs
- callcounting.h: Documentation clarifying precode lifecycle invariant
- prestub.cpp: DoBackpatch comment documenting the two-state precode rule