Restore support for include_directories() in toolchain files

Any include_directories() calls in toolchain files are used during our ABI detection step even though it does not include any system headers. Since !2716 (merged), that check is also used to detect implicit include directories. Any include_directories() in a toolchain file are detected as implicit and later excluded from explicit specification on compiler command lines, thus breaking the purpose of the calls in the first place.

Fix the implicit include directory detection step to avoid using paths from include_directories() calls in the toolchain file.

Fixes: #19079 (closed)