[android] Escape include paths in Android.

[drodriguez]

Before the result of _add_variant_c_compile_flags was a string, so appending several "-isystem" was not a problem. With #29451 the rules have changed since the list is now handled by CMake, and it deduplicates the repeated members in the list. Thanks, CMake.

Should fix the Android CI builds that were failing since the merging of #29451.

[drodriguez]

Should not affect Darwin, Linux, nor Windows.

@swift-ci please smoke test

[compnerd]

Personally, I think that we should split this into the three sites where it is used and use

swift_android_libcxx_include_paths(ANDROID_CXX_INCLUDES)
swift_android_include_for_arch("${..._ARCHITECTURE}"
  "ANDROID_${..._ARCHITECTURE}_INCLUDE")
target_include_directories(<TARGET> SYSTEM PRIVATE
  ${ANDROID_CXX_INCLUDES}
  ${ANDROID_${...ARCHITECTURE}_INCLUDE})

But, doing this to fix the bots first is fine.

[drodriguez]

Yes, I think _add_variant_c_compile_flags and _add_variant_c_linker_flags should manipulate the target directly, instead of returning values. It will make the functions easier to think about.