Add FileActionSymlink and `llb.Symlink` #5681

Job	Run time
build (windows/amd64)	1m 46s
sandbox-build (linux/amd64)	6m 42s
sandbox-build (linux/arm64)	6m 40s
build (freebsd/amd64)	1m 39s
test-freebsd-amd64	2m 47s
test-windows-amd64 (containerd, ./client)	7m 15s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 38s
test-windows-amd64 (containerd, ./worker/containerd)	3m 19s
test-windows-amd64 (containerd, ./solver)	3m 26s
test-windows-amd64 (containerd, ./frontend)	3m 39s
test-windows-amd64 (containerd, ./frontend/dockerfile)	39m 23s
test-windows-amd64 (./..., 1)	3m 47s
	1h 23m 1s

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