also precompile for SubString (#542)

Author: KristofferC

src/precompile.jl

@@ -6,6 +6,7 @@ let filename = joinpath(@__DIR__, "literal_parsing.jl")
     if _has_v1_6_hooks
         enable_in_core!()
         Meta.parse("1 + 2")
+        Meta.parse(SubString("1 + 2"))
         enable_in_core!(false)
     end
 end