effects: taint :nothrow effect on unknown :static_parameter (conservatively)

With this commit, we taint `:nothrow` effect property correctly on
access to unknown `:static_parameter`, e.g.:
```julia
unknown_sparam_throw(::Union{Nothing, Type{T}}) where T = (T; nothing)
@test Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, ((Type{Int},))))
@test !Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, ((Nothing,))))
```

This commit implements a very conservative analysis, and thus there is a
room for improvement still, e.g.:
```julia
unknown_sparam_nothrow(x::Ref{T}) where {T} = (T; nothing)
@test_broken Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_nothrow, (Ref,)))
```

Author: aviatesk

base/compiler/abstractinterpretation.jl

@@ -2192,6 +2192,10 @@ function abstract_eval_value_expr(interp::AbstractInterpreter, e::Expr, vtypes::
         if 1 <= n <= length(sv.sptypes)
             rt = sv.sptypes[n]
         end
+        if !isa(rt, Const)
+            merge_effects!(interp, sv, Effects(EFFECTS_TOTAL; nothrow=false))
+        end
+        return rt
     elseif head === :boundscheck
         if isa(sv, InferenceState)
             # If there is no particular `@inbounds` for this function, then we only taint `:noinbounds`,

test/compiler/effects.jl

@@ -743,3 +743,13 @@ end
 
 @test Base.ismutationfree(Type{Union{}})
 @test Core.Compiler.is_total(Base.infer_effects(typejoin, ()))
+
+
+# unknown :static_parameter should taint :nothrow
+# https://github.com/JuliaLang/julia/issues/46771
+unknown_sparam_throw(::Union{Nothing, Type{T}}) where T = (T; nothing)
+@test Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Type{Int},)))
+@test !Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Nothing,)))
+
+unknown_sparam_nothrow(x::Ref{T}) where {T} = (T; nothing)
+@test_broken Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_nothrow, (Ref,)))