Fix despecialisation of annotations list

In f117a50 code modifying annotations via list comprehensions was
introduced. However, when (1) there are annotations present and (2) all
annotations are filtered out, this comprehension is inferred to a wider
type incompatible with the AnnotatedString constructor. This can be
fixed by changing the first element of the tuple to directly use the
UnitRange{Int} constructor, which stops it being inferred as an Any.

base/strings/annotated.jl

@@ -486,7 +486,7 @@ function read(io::AnnotatedIOBuffer, ::Type{AnnotatedString{T}}) where {T <: Abs
     if (start = position(io)) == 0
         AnnotatedString(read(io.io, T), copy(io.annotations))
     else
-        annots = [(max(1, first(region) - start):last(region)-start, val)
+        annots = [(UnitRange{Int}(max(1, first(region) - start), last(region)-start), val)
                   for (region, val) in io.annotations if last(region) > start]
         AnnotatedString(read(io.io, T), annots)
     end

test/strings/annotated.jl

@@ -122,6 +122,7 @@ end
     @test read(seek(aio, 1), Base.AnnotatedString) == Base.AnnotatedString("ello world", [(1:4, :tag => 1), (6:10, :tag => 2)])
     @test read(seek(aio, 4), Base.AnnotatedString) == Base.AnnotatedString("o world", [(1:1, :tag => 1), (3:7, :tag => 2)])
     @test read(seek(aio, 5), Base.AnnotatedString) == Base.AnnotatedString(" world", [(2:6, :tag => 2)])
+    @test read(aio, Base.AnnotatedString) == Base.AnnotatedString("")
     @test read(seekstart(truncate(deepcopy(aio), 5)), Base.AnnotatedString) == Base.AnnotatedString("hello", [(1:5, :tag => 1)])
     @test read(seekstart(truncate(deepcopy(aio), 6)), Base.AnnotatedString) == Base.AnnotatedString("hello ", [(1:5, :tag => 1)])
     @test read(seekstart(truncate(deepcopy(aio), 7)), Base.AnnotatedString) == Base.AnnotatedString("hello w", [(1:5, :tag => 1), (7:7, :tag => 2)])