Add FileActionSymlink and `llb.Symlink` #5742

Job	Run time
build (windows/amd64)	1m 54s
build (freebsd/amd64)	1m 58s
sandbox-build (linux/amd64)	6m 53s
sandbox-build (linux/arm64)	6m 41s
test-freebsd-amd64	2m 31s
test-windows-amd64 (containerd, ./client)	6m 59s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 39s
test-windows-amd64 (containerd, ./worker/containerd)	2m 24s
test-windows-amd64 (containerd, ./solver)	3m 47s
test-windows-amd64 (containerd, ./frontend)	3m 4s
test-windows-amd64 (containerd, ./frontend/dockerfile)	37m 58s
test-windows-amd64 (./..., 1)	3m 51s
	1h 20m 39s

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