cconvert(Ref{BigFloat}, x) should return BigFloatData (#57367)

#55906 changed `cconvert(Ref{BigFloat}, x::BigFloat)` to return `x.d`,
but neglected to do so for other types of `x`, where it still returns a
`Ref{BigFloat}` and hence is now returning the wrong type for `ccall`.

Not only does this break backwards compatibility
(JuliaMath/SpecialFunctions.jl#485), but it
also seems simply wrong: the *whole* job of `cconvert` is to convert
objects to the correct type for use with `ccall`. This PR does so (at
least for `Number` and `Ref{BigFloat}`).

(cherry picked from commit 6dca4f4)

Author: stevengj

base/mpfr.jl

@@ -211,6 +211,8 @@ end
 Base.unsafe_convert(::Type{Ref{BigFloat}}, x::Ptr{BigFloat}) = error("not compatible with mpfr")
 Base.unsafe_convert(::Type{Ref{BigFloat}}, x::Ref{BigFloat}) = error("not compatible with mpfr")
 Base.cconvert(::Type{Ref{BigFloat}}, x::BigFloat) = x.d # BigFloatData is the Ref type for BigFloat
+Base.cconvert(::Type{Ref{BigFloat}}, x::Number) = convert(BigFloat, x).d # avoid default conversion to Ref(BigFloat(x))
+Base.cconvert(::Type{Ref{BigFloat}}, x::Ref{BigFloat}) = x[].d
 function Base.unsafe_convert(::Type{Ref{BigFloat}}, x::BigFloatData)
     d = getfield(x, :d)
     p = Base.unsafe_convert(Ptr{Limb}, d)

test/mpfr.jl

@@ -1097,3 +1097,10 @@ end
         end
     end
 end
+
+# BigFloatData is the Ref type for BigFloat in ccall:
+@testset "cconvert(Ref{BigFloat}, x)" begin
+    for x in (1.0, big"1.0", Ref(big"1.0"))
+        @test Base.cconvert(Ref{BigFloat}, x) isa Base.MPFR.BigFloatData
+    end
+end