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: swolchok

examples/models/llama/runner/CMakeLists.txt

@@ -38,7 +38,7 @@ include(${EXECUTORCH_SRCS_FILE})
 list(TRANSFORM _llama_runner__srcs PREPEND "${EXECUTORCH_ROOT}/")
 
 target_include_directories(
-  extension_module INTERFACE ${_common_include_directories} ${EXECUTORCH_INCLUDE_DIRS}
+  extension_module INTERFACE ${_common_include_directories}
 )
 
 list(
@@ -82,6 +82,6 @@ set(llama_runner_deps executorch extension_data_loader extension_module
 target_link_libraries(llama_runner PUBLIC ${llama_runner_deps})
 
 target_include_directories(
-  llama_runner INTERFACE ${_common_include_directories} ${EXECUTORCH_INCLUDE_DIRS}
+  llama_runner INTERFACE ${_common_include_directories} ${EXECUTORCH_ROOT}
 )
 target_compile_options(llama_runner PUBLIC ${_preprocessor_flag})