Allow for :foreigncall to transition to GC safe automatically (JuliaLang#49933)

This has been bouncing around as a idea for a while.
One of the challenges around time-to-safepoint has been Julia code
that is calling libraries. 

Since foreign code will not include safepoints we see increased latency
when one thread is running a foreign-call and another wants to trigger
GC.

The open design question here is:
- Do we expose this as an option the user must "opt-in", e.g. by using a
  keyword arg to `@ccall` or a specific calling-convetion.
- Or do we turn this on for all ccall, except for Julia runtime calls.

There is relativly little code outside the Julia runtime that needs to
be "GC unsafe",
exception are programs that directly use the Julia C-API. Incidentially
`jl_adopt_thread`
and `@cfunction`/`@ccallable` do the right thing and transition to "GC
unsafe", regardless
of what state the thread currently is in.

I still need to figure out how to reliably detect Julia runtime calls,
but I think we can
switch all other calls to "GC safe". We should also consider
optimizations that mark large
regions of code without Julia runtime interactions as "GC safe" in
particular numeric
for-loops.

Closes JuliaLang#57057

---------

Co-authored-by: Gabriel Baraldi <baraldigabriel@gmail.com>

Author: vchuravy

src/abstractinterpretation.jl

@@ -3421,7 +3421,7 @@ function abstract_eval_foreigncall(interp::AbstractInterpreter, e::Expr, sstate:
         abstract_eval_value(interp, x, sstate, sv)
     end
     cconv = e.args[5]
-    if isa(cconv, QuoteNode) && (v = cconv.value; isa(v, Tuple{Symbol, UInt16}))
+    if isa(cconv, QuoteNode) && (v = cconv.value; isa(v, Tuple{Symbol, UInt16, Bool}))
         override = decode_effects_override(v[2])
         effects = override_effects(effects, override)
     end

src/validation.jl

@@ -23,7 +23,7 @@ const VALID_EXPR_HEADS = IdDict{Symbol,UnitRange{Int}}(
     :meta => 0:typemax(Int),
     :global => 1:1,
     :globaldecl => 1:2,
-    :foreigncall => 5:typemax(Int), # name, RT, AT, nreq, (cconv, effects), args..., roots...
+    :foreigncall => 5:typemax(Int), # name, RT, AT, nreq, (cconv, effects, gc_safe), args..., roots...
     :cfunction => 5:5,
     :isdefined => 1:2,
     :code_coverage_effect => 0:0,