Fix inconsistency between exct and nothrow effect

Fixes #52846. Different unreachability checks use different definitions for
this and were inconsistenct, causing the assertion.

Since the exct modeling isn't fully plubmed through all corners of the
compiler yet, also change the caller code to force the nothrow effect
to be authoritative for the time being.

base/compiler/abstractinterpretation.jl

@@ -2119,8 +2119,14 @@ function abstract_call_known(interp::AbstractInterpreter, @nospecialize(f),
             abstract_call_gf_by_type(interp, f, ArgInfo(nothing, T), si, atype, sv, max_methods)
         end
         pT = typevar_tfunc(𝕃ᵢ, n, lb_var, ub_var)
-        effects = builtin_effects(𝕃ᵢ, Core._typevar, Any[n, lb_var, ub_var], pT)
-        return CallMeta(pT, Any, effects, call.info)
+        typevar_argtypes = Any[n, lb_var, ub_var]
+        effects = builtin_effects(𝕃ᵢ, Core._typevar, typevar_argtypes, pT)
+        if effects.nothrow
+            exct = Union{}
+        else
+            exct = builtin_exct(𝕃ᵢ, Core._typevar, typevar_argtypes, pT)
+        end
+        return CallMeta(pT, exct, effects, call.info)
     elseif f === UnionAll
         call = abstract_call_gf_by_type(interp, f, ArgInfo(nothing, Any[Const(UnionAll), Any, Any]), si, Tuple{Type{UnionAll}, Any, Any}, sv, max_methods)
         return abstract_call_unionall(interp, argtypes, call)
@@ -3288,10 +3294,12 @@ function typeinf_local(interp::AbstractInterpreter, frame::InferenceState)
             # Process non control-flow statements
             (; changes, rt, exct) = abstract_eval_basic_statement(interp,
                 stmt, currstate, frame)
-            if exct !== Union{}
-                update_exc_bestguess!(interp, exct, frame)
-            end
             if !has_curr_ssaflag(frame, IR_FLAG_NOTHROW)
+                if exct !== Union{}
+                    update_exc_bestguess!(interp, exct, frame)
+                    # TODO: assert that these conditions match. For now, we assume the `nothrow` flag
+                    # to be correct, but allow the exct to be an over-approximation.
+                end
                 propagate_to_error_handler!(currstate, frame, 𝕃ᵢ)
             end
             if rt === Bottom