Add FileActionSymlink and `llb.Symlink` #5498

Job	Run time
build (windows/amd64)	1m 50s
build (freebsd/amd64)	1m 47s
test-windows-amd64 (containerd, ./client)	5m 26s
test-freebsd-amd64	1h 43m 51s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 1s
test-windows-amd64 (containerd, ./worker/containerd)	3m 21s
test-windows-amd64 (containerd, ./solver)	2m 38s
test-windows-amd64 (containerd, ./frontend)	3m 18s
test-windows-amd64 (containerd, ./frontend/dockerfile)	35m 51s
test-windows-amd64 (./..., 1)	4m 9s
	2h 44m 12s

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