Fix apply_type_tfunc for Union{T::TypeVar}

The type parameters to `Union` may be `Type`s or `TypeVar`s, but
`apply_type_tfunc` failed to recognize the latter as valid in the
single-argument case.

(cherry picked from commit fd79b58)

base/compiler/tfuncs.jl

@@ -1541,7 +1541,7 @@ function apply_type_tfunc(@nospecialize(headtypetype), @nospecialize args...)
             end
         end
         if largs == 1 # Union{T} --> T
-            u1 = typeintersect(widenconst(args[1]), Type)
+            u1 = typeintersect(widenconst(args[1]), Union{Type,TypeVar})
             valid_as_lattice(u1) || return Bottom
             return u1
         end

test/compiler/inference.jl

@@ -2364,9 +2364,9 @@ end |> only === Int
 # Equivalence of Const(T.instance) and T for singleton types
 @test Const(nothing) ⊑ Nothing && Nothing ⊑ Const(nothing)
 
-# Don't pessimize apply_type to anything worse than Type and yield Bottom for invalid Unions
+# Don't pessimize apply_type to anything worse than Type (or TypeVar) and yield Bottom for invalid Unions
 @test only(Base.return_types(Core.apply_type, Tuple{Type{Union}})) == Type{Union{}}
-@test only(Base.return_types(Core.apply_type, Tuple{Type{Union},Any})) == Type
+@test only(Base.return_types(Core.apply_type, Tuple{Type{Union},Any})) == Union{Type,TypeVar}
 @test only(Base.return_types(Core.apply_type, Tuple{Type{Union},Any,Any})) == Type
 @test only(Base.return_types(Core.apply_type, Tuple{Type{Union},Int})) == Union{}
 @test only(Base.return_types(Core.apply_type, Tuple{Type{Union},Any,Int})) == Union{}
@@ -4277,3 +4277,6 @@ end
 # Issue #46839: `abstract_invoke` should handle incorrect call type
 @test only(Base.return_types(()->invoke(BitSet, Any, x), ())) === Union{}
 @test only(Base.return_types(()->invoke(BitSet, Union{Tuple{Int32},Tuple{Int64}}, 1), ())) === Union{}
+
+# issue #48374
+@test (() -> Union{<:Nothing})() == Nothing