Add FileActionSymlink and `llb.Symlink` #5083

Job	Run time
prepare	1s
test / prepare	2m 12s
test / run (containerd, ./frontend/dockerfile, integration)	5m 9s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 21s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	6m 9s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 41s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 3s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 29s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	5m 5s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 29s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 6s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 5s
test / run (oci, ./frontend/dockerfile, integration)	4m 34s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 2s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 15s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 57s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 53s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 52s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 28s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 37s
image (mainline)	5m 26s
image (labs)	5m 36s
scout	0s
release	0s
	1h 59m 30s

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