inference: Guard TypeVar special case against vararg (JuliaLang#52721)

Fix JuliaLang#52613 by making the TypeVar special case in inference check for
vararg first. There's nothing the special case can really do with vararg
anyway, so fall back to the ordinary abstract call handling.

test/compiler/inference.jl

@@ -5130,5 +5130,10 @@ let TV = TypeVar(:T)
     @test abstract_call_unionall_vararg(some) isa UnionAll
 end
 
+# Issue #51927
+let 𝕃 = Core.Compiler.fallback_lattice
+    @test apply_type_tfunc(𝕃, Const(Tuple{Vararg{Any,N}} where N), Int) == Type{NTuple{_A, Any}} where _A
+end
+
 # Issue #52613
 @test (code_typed((Any,)) do x; TypeVar(x...); end)[1][2] === TypeVar