Deprecate typesafe variadic class arguments

[dkorpel]

Testing it as an error first to see if anything on buildkite relies on this feature.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#20673"

[dkorpel]

2 buildkite projects fail when it becomes an error:

ddox:

[main(----) INF] Generating module: test
+ '[' '!' -f .no_diff ']'
+ git --no-pager diff --exit-code -- docs
+ popd
~/rejectedsoftware-ddox
+ for dir in tests/*
+ pushd tests/issue177_variadic
~/rejectedsoftware-ddox/tests/issue177_variadic ~/rejectedsoftware-ddox
+ dmd -Xftest.json -Dd__dummy_html -c -o- test.d
test.d(19): Error: cannot have typesafe variadic parameter of class type `test.C`
void bug(scope C c...)
     ^
test.d(40): Error: cannot have typesafe variadic parameter of class type `test.C`
alias FT3 = void function(scope C c...);
^
test.d(55): Error: cannot have typesafe variadic parameter of class type `test.C`
void function(scope C c...) var3;

This is simply a test case for all kinds of typesafe variadic functions, easily removed.

dwt:

    Starting Performing "debug" build using /buildkite/builds/zaphod-4/dlang/dmd/buildkite-ci-build/distribution/bin/dmd for x86_64.
    Building dwt ~master: building configuration [linux-gtk]
base/src/java/lang/Class.d(42,17): Error: cannot have typesafe variadic parameter of class type `java.lang.Class.Class`
    Constructor getConstructor(Class parameterTypes...){
                ^
base/src/java/lang/Class.d(54,17): Error: cannot have typesafe variadic parameter of class type `java.lang.Class.Class`
    Constructor getDeclaredConstructor(Class parameterTypes...){
                ^
base/src/java/lang/Class.d(70,12): Error: cannot have typesafe variadic parameter of class type `java.lang.Class.Class`
    Method getDeclaredMethod(String name, Class parameterTypes...){
           ^
Error /buildkite/builds/zaphod-4/dlang/dmd/buildkite-ci-build/distribution/bin/dmd failed with exit code 1.

These 3 functions are stubs in root class Class that call implMissing, and are never overridden as far as I can see. Considering Class doesn't even have any fields, it's probably a typo, and meant to say Class[] parameterTypes....