Update on "Use c10 version of half/bfloat16 in executorch"

Accomplished by importing relevant files from c10 into
executorch/runtime/core/portable_type/c10, and then using `using` in
the top-level ExecuTorch headers. This approach should keep the
ExecuTorch build hermetic for embedded use cases. In the future, we
should add a CI job to ensure the c10 files stay identical to the
PyTorch ones.

Differential Revision: [D66106969](https://our.internmc.facebook.com/intern/diff/D66106969/)

[ghstack-poisoned]

Author: Github Executorch

examples/apple/coreml/executor_runner/coreml_executor_runner.xcodeproj/project.pbxproj

@@ -214,6 +214,7 @@
 				GCC_OPTIMIZATION_LEVEL = 0;
 				GCC_PREPROCESSOR_DEFINITIONS = (
 					"DEBUG=1",
+                                	"C10_USING_CUSTOM_GENERATED_MACROS",
 					"$(inherited)",
 				);
 				GCC_WARN_64_TO_32_BIT_CONVERSION = YES;
@@ -271,6 +272,10 @@
 				ENABLE_USER_SCRIPT_SANDBOXING = YES;
 				GCC_C_LANGUAGE_STANDARD = gnu17;
 				GCC_NO_COMMON_BLOCKS = YES;
+				GCC_PREPROCESSOR_DEFINITIONS = (
+                                	"C10_USING_CUSTOM_GENERATED_MACROS",
+					"$(inherited)",
+				);
 				GCC_WARN_64_TO_32_BIT_CONVERSION = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES_ERROR;
 				GCC_WARN_UNDECLARED_SELECTOR = YES;
@@ -291,7 +296,10 @@
 				CODE_SIGN_STYLE = Automatic;
 				DEVELOPMENT_TEAM = "";
 				ENABLE_HARDENED_RUNTIME = YES;
-				HEADER_SEARCH_PATHS = "$(SRCROOT)/include";
+				HEADER_SEARCH_PATHS = (
+                                	"$(SRCROOT)/include",
+                                	"$(SRCROOT)/include/executorch/runtime/core/portable_type",
+                                );
 				IPHONEOS_DEPLOYMENT_TARGET = 16.0;
 				LIBRARY_SEARCH_PATHS = (
 					"$(SRCROOT)/libraries",
@@ -310,7 +318,10 @@
 				CODE_SIGN_STYLE = Automatic;
 				DEVELOPMENT_TEAM = "";
 				ENABLE_HARDENED_RUNTIME = YES;
-				HEADER_SEARCH_PATHS = "$(SRCROOT)/include";
+				HEADER_SEARCH_PATHS = (
+                                	"$(SRCROOT)/include",
+                                	"$(SRCROOT)/include/executorch/runtime/core/portable_type",
+                                );
 				IPHONEOS_DEPLOYMENT_TARGET = 16.0;
 				LIBRARY_SEARCH_PATHS = (
 					"$(SRCROOT)/libraries",