CMake: Use toolchain info for system includes

CMake already provides `CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES`, so there's
no need to invoke the compiler to determine system include directories.

For the Clang tool based header generator, adding the includes from the
current toolchain is strictly more correct than having it try and figure them
out itself (which will favor files from any installed libc++ and/or Clang).

PiperOrigin-RevId: 531207747
Change-Id: Icfcf7b495a0a5782c21a665984d9039d365db898

cmake/SapiBuildDefs.cmake

@@ -153,10 +153,13 @@ function(add_sapi_library)
     "--sapi_ns=${_sapi_NAMESPACE}"
   )
   if(SAPI_ENABLE_CLANG_TOOL)
+    set(_sapi_isystem_args ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})
+    list(TRANSFORM _sapi_isystem_args PREPEND --extra-arg-before=-isystem)
     list(APPEND _sapi_generator_command
       sapi_generator_tool
       -p "${CMAKE_CURRENT_BINARY_DIR}"
       ${_sapi_generator_args}
+      ${_sapi_isystem_args}
       ${_sapi_full_inputs}
     )
     add_custom_command(
@@ -179,9 +182,8 @@ function(add_sapi_library)
     add_custom_command(
       OUTPUT "${_sapi_gen_header}" "${_sapi_isystem}"
       COMMAND sh -c
-              "${CMAKE_CXX_COMPILER} -E -x c++ -v /dev/null 2>&1 | \
-               awk '/> search starts here:/{f=1;next}/^End of search/{f=0}f{print $1}' \
-               > \"${_sapi_isystem}\""
+              "printf '${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES}' | \
+               sed \"s/;/\\n/g\" > \"${_sapi_isystem}\""
       COMMAND ${_sapi_generator_command}
       COMMENT "Generating interface"
       DEPENDS ${_sapi_INPUTS}