Compile out RCTGetModuleClasses/RCTRegisterModule when both legacy interops are removed (#57670)

[christophpurrer]

Summary:

Changelog: [iOS][Changed] - Compile out RCTGetModuleClasses/RCTRegisterModule and skip static module registration when both RCT_REMOVE_LEGACY_MODULE_INTEROP and RCT_REMOVE_LEGACY_COMPONENT_INTEROP are defined

RCTGetModuleClasses() and the RCTModuleClasses registry it reads (populated by
RCTRegisterModule) exist solely to support the two legacy interop layers. Every reader
of the registry is already gated behind either RCT_REMOVE_LEGACY_MODULE_INTEROP (TurboModule
⇄ legacy NativeModule interop) or RCT_REMOVE_LEGACY_COMPONENT_INTEROP (Fabric ⇄ legacy
ViewManager interop). When an app defines both macros, the registry has no readers and the
function, its backing statics, and RCTRegisterModule are dead code.

This diff compiles them out in that configuration:

• React/Base/RCTBridge.mm: guard RCTModuleClasses/RCTModuleClassesSyncQueue,
  RCTGetModuleClasses(), getCoreModuleClasses(), and RCTRegisterModule() behind
  #if !defined(RCT_REMOVE_LEGACY_MODULE_INTEROP) || !defined(RCT_REMOVE_LEGACY_COMPONENT_INTEROP).
  The old-arch warning helpers (getModulesLoadedWithOldArch et al.) stay compiled in — they have
  an external caller in RCTInstance.mm — and simply observe an empty list once the registry's only
  writer is gone.
• React/Base/RCTBridge+Private.h: guard the matching RCT_EXTERN declarations.
• React/Base/RCTBridgeModule.h: add a "both-removed" variant of RCT_EXPORT_MODULE /
  RCT_EXPORT_MODULE_NO_LOAD that emits only +moduleName and omits the RCTRegisterModule call,
  so native modules still link when the symbol is gone.
• RCTLegacyUIManagerConstantsProvider.{h,mm}: wrap the file body in
  #ifndef RCT_REMOVE_LEGACY_COMPONENT_INTEROP, matching the sibling LegacyViewManagerInterop*
  files. It was the one caller of RCTGetModuleClasses whose file body was not already guarded.
• fbobjc/Libraries/FBReactModuleRegistration/FBReactModuleRegistration.mm: the FIT/new-arch path
  (RCT_REMOVE_LEGACY_ARCH) hand-calls RCTRegisterModule. On cove-ios, which sets all three
  macros, that symbol is now compiled out, so the call is additionally gated to only run when
  RCTRegisterModule still exists. The registry has no readers in that config, so this is a no-op.

The default build (neither/one macro set) is behavior-unchanged: all guarded code is still compiled in.

Differential Revision: D113555374

[meta-codesync]

@christophpurrer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D113555374.