SecretBuffer finalizer warning errors out with multiple tasks

[staticfloat]

It looks like the helpful warning inside of the SecretBuffer finalizer causes multitasking to throw an error. To reproduce this, create a second task, then cause a SecretBuffer to be GC collected, triggering a warning being printed, followed by an error that task switching is not allowed within GC finalizers:

julia> @async begin
       sleep(10)
       println("Done!")
       end

       z = Base.SecretBuffer("foo")
       z = nothing
       GC.gc()
┌ Warning: a SecretBuffer was `shred!`ed by the GC; use `shred!` manually after use to minimize exposure.
└ @ Base secretbuffer.jl:169
┌ Error: Exception while generating log record in module Base at secretbuffer.jl:169
│   exception =
│    task switch not allowed from inside gc finalizer
│    Stacktrace:
│     [1] try_yieldto(::typeof(Base.ensure_rescheduled), ::Base.RefValue{Task}) at ./task.jl:602
│     [2] wait() at ./task.jl:668
│     [3] uv_write(::Base.TTY, ::Ptr{UInt8}, ::UInt64) at ./stream.jl:935
│     [4] unsafe_write(::Base.TTY, ::Ptr{UInt8}, ::UInt64) at ./stream.jl:1007
│     [5] unsafe_write at ./io.jl:593 [inlined]
│     [6] macro expansion at ./gcutils.jl:91 [inlined]
│     [7] write(::Base.TTY, ::Array{UInt8,1}) at ./io.jl:616
│     [8] #handle_message#2(::Nothing, ::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(Base.CoreLogging.handle_message), ::Logging.ConsoleLogger, ::Base.CoreLogging.LogLevel, ::String, ::Module, ::String, ::Symbol, ::String, ::Int64) at /Users/sabae/tmp/julia-build/julia-release-1.3/usr/share/julia/stdlib/v1.3/Logging/src/ConsoleLogger.jl:161
│     [9] handle_message at /Users/sabae/tmp/julia-build/julia-release-1.3/usr/share/julia/stdlib/v1.3/Logging/src/ConsoleLogger.jl:100 [inlined]
│     [10] macro expansion at ./logging.jl:320 [inlined]
│     [11] final_shred!(::Base.SecretBuffer) at ./secretbuffer.jl:169
│     [12] gc at ./gcutils.jl:60 [inlined] (repeats 2 t
imes)
│     [13] top-level scope at REPL[1]:8
│     [14] eval(::Module, ::Any) at ./boot.jl:330
│     [15] eval_user_input(::Any, ::REPL.REPLBackend) at /Users/sabae/tmp/julia-build/julia-release-1.3/usr/share/julia/stdlib/v1.3/REPL/src/REPL.jl:86
│     [16] macro expansion at /Users/sabae/tmp/julia-build/julia-release-1.3/usr/share/julia/stdlib/v1.3/REPL/src/REPL.jl:118 [inlined]
│     [17] (::REPL.var"#26#27"{REPL.REPLBackend})() at ./task.jl:333
└ @ Base secretbuffer.jl:169
Done!

Perhaps the most infuriating thing is that it appears that everything worked properly, despite the fatal error.

[c42f]

Oh ick. Somewhat related to #26687 and logging from other special contexts.