nixd/Sema: add CompletionBuilder #1418

Job	Run time
nix-build-develop-clang (none, release, true)	4m 26s
nix-build-develop-clang (none, release, false)	5m 35s
nix-build-develop-clang (none, debug, true)	3m 49s
nix-build-develop-clang (none, debug, false)	3m 46s
nix-build-develop-gcc (none, release, true)	5m 17s
nix-build-develop-gcc (none, release, false)	4m 51s
nix-build-develop-gcc (none, debug, true)	4m 32s
nix-build-develop-gcc (none, debug, false)	6m 21s
nix-build-develop-gcc (undefined, release, true)	13m 14s
nix-build-develop-gcc (undefined, release, false)	13m 51s
nix-build-develop-gcc (undefined, debug, true)	5m 26s
nix-build-develop-gcc (undefined, debug, false)	7m 14s
	1h 18m 22s

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