Could it be? Of course, the upcoming Swift 5.6 compiler is the first to link against the Swift standard library, a result of @Erik_Eckstein's ongoing libswift effort which you can read about here:
> readelf -d swift-5.6-DEVELOPMENT-SNAPSHOT-2022-01-11-a-centos8/usr/bin/swift | ag swift
0x0000000000000001 (NEEDED) Shared library: [libswiftCore.so]
0x000000000000001d (RUNPATH) Library runpath: [$ORIGIN/../lib/swift/linux]
It seems to mostly consist of Swift wrappers around optimization passes written in C++ right now. I know it was enabled because I had to patch it to build for my native Swift toolchain for Android, so I could get the first Swift 5.6 snapshot from last week running (a working Android AArch64 Swift 5.6 toolchain for the Termux app is available as a github Action CI artifact).
Will it be? The tokei code count program reports 18.3 klocs of C++ in lib/Parse, it all depends on if someone sees fit to rewrite that in Swift.