Add FileActionSymlink and `llb.Symlink` #5506

Job	Run time
build (windows/amd64)	2m 7s
build (freebsd/amd64)	1m 39s
test-windows-amd64 (containerd, ./client)	4m 17s
test-freebsd-amd64	3m 6s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 49s
test-windows-amd64 (containerd, ./worker/containerd)	2m 49s
test-windows-amd64 (containerd, ./solver)	1m 46s
test-windows-amd64 (containerd, ./frontend)	2m 16s
test-windows-amd64 (containerd, ./frontend/dockerfile)	33m 43s
test-windows-amd64 (./..., 1)	4m 11s
	58m 43s

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