Retain usage of HERMES_ENABLE_DEBUGGER flag (#35646)

Summary:
Pull Request resolved: #35646. Even though the RN code itself does not need `HERMES_ENABLE_DEBUGGER` anymore since the `ifdef`s are gone, the Hermes' headers it includes *do* still need that flag. This is because they were compiled with `HERMES_ENABLE_DEBUGGER` to set true, so that compile flag must match later on when importing the headers.

Changelog:
[Internal][Fixed] - We erroneously deleted usage of the HERMES_ENABLE_DEBUGGER flag, add it back.

Reviewed By: cortinico, mattbfb

Differential Revision: D42041827

fbshipit-source-id: db875f40a067c09aba37ab1b07422157f63b3700

ReactCommon/hermes/React-hermes.podspec

@@ -42,7 +42,7 @@ Pod::Spec.new do |s|
   s.pod_target_xcconfig    = {
                                "HEADER_SEARCH_PATHS" => "\"${PODS_ROOT}/hermes-engine/destroot/include\" \"$(PODS_TARGET_SRCROOT)/..\" \"$(PODS_ROOT)/boost\" \"$(PODS_ROOT)/RCT-Folly\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/libevent/include\"",
                                "CLANG_CXX_LANGUAGE_STANDARD" => "c++17"
-                             }
+                             }.merge!(build_type == :debug ? { "GCC_PREPROCESSOR_DEFINITIONS" => "HERMES_ENABLE_DEBUGGER=1" } : {})
   s.header_dir             = "reacthermes"
   s.dependency "React-cxxreact", version
   s.dependency "React-jsiexecutor", version