Manually split `string(a::Union{Char, String, SubString{String}, Symb…

…ol}...)` (#43939)

* Manually split `string(a::Union{Char, String, SubString{String}, Symbol}...)`

4 is one too many for automatic Union-splitting.  Poor inference in
this method accounts for more than 1000 invalidations.

base/strings/substring.jl

@@ -214,10 +214,16 @@ end
 function string(a::Union{Char, String, SubString{String}, Symbol}...)
  n = 0
  for v in a
+ # 4 types is too many for automatic Union-splitting, so we split manually
+ # and allow one specializable call site per concrete type
  if v isa Char
  n += ncodeunits(v)
- else
+ elseif v isa String
+ n += sizeof(v)
+ elseif v isa SubString{String}
  n += sizeof(v)
+ else
+ n += sizeof(v::Symbol)
  end
  end
  out = _string_n(n)