Use Base.TOML.Parser{Dates} for TOML parsing w/ Dates support (#3938)

* Use `Base.TOML.Parser{Dates}` for TOML parsing w/ Dates support

This will allow us to delete the `.Dates` field upstream.

* Use `Base.TOMLCache(p::TOML.Parser)` constructor

These were relying on implicit `convert()` behavior in the previous
constructor, which is harmless in this case but best not to depend upon.

---------

Co-authored-by: Ian Butterworth <i.r.butterworth@gmail.com>
(cherry picked from commit e4a6723)

src/Registry/registry_instance.jl

@@ -1,5 +1,6 @@
 using Base: UUID, SHA1
 using TOML
+using Dates
 using Tar
 using ..Versions: VersionSpec, VersionRange
 
@@ -15,19 +16,15 @@ function to_tar_path_format(file::AbstractString)
 end
 
 # See loading.jl
-const TOML_CACHE = let parser = Base.TOML.Parser()
-    parser.Dates = Dates
-    Base.TOMLCache(parser, Dict{String, Dict{String, Any}}())
-end
+const TOML_CACHE = Base.TOMLCache(Base.TOML.Parser{Dates}())
 const TOML_LOCK = ReentrantLock()
 _parsefile(toml_file::AbstractString) = Base.parsed_toml(toml_file, TOML_CACHE, TOML_LOCK)
 function parsefile(in_memory_registry::Union{Dict, Nothing}, folder::AbstractString, file::AbstractString)
     if in_memory_registry === nothing
         return _parsefile(joinpath(folder, file))
     else
         content = in_memory_registry[to_tar_path_format(file)]
-        parser = Base.TOML.Parser(content; filepath=file)
-        parser.Dates = Dates
+        parser = Base.TOML.Parser{Dates}(content; filepath=file)
         return Base.TOML.parse(parser)
     end
 end

src/Types.jl

@@ -53,10 +53,7 @@ function deepcopy_toml(x::Dict{String, Any})
 end
 
 # See loading.jl
-const TOML_CACHE = let parser = Base.TOML.Parser()
-    parser.Dates = Dates
-    Base.TOMLCache(parser, Dict{String, Dict{String, Any}}())
-end
+const TOML_CACHE = Base.TOMLCache(Base.TOML.Parser{Dates}())
 const TOML_LOCK = ReentrantLock()
 # Some functions mutate the returning Dict so return a copy of the cached value here
 parse_toml(toml_file::AbstractString) =