unique: string arguments do not need to escape

[mknyszek]

unique clones any strings it finds in its input, if it can find them statically. (strings hidden somewhere in an interface value are excepted.) This means string arguments (or types whose only reference types are strings) never need to escape.

Note that pointer arguments still need to escape, because stack objects have no stable identity. But string arguments' identity is their data.

[jakebailey]

Funnily this is effectively what I was fixing in CL 671955, mistakenly thinking I was fixing #71926.

[gopherbot]

Change https://go.dev/cl/671955 mentions this issue: cmd/compile: do not escape strings passed to unique.Make