fix new type instability from getindex(::String, r::UnitRange{Int})

After investigating the cause of the invalidation reported in
#55583, it was found that the issue arises only when `r`
is propagated as an extended lattice element, such as
`PartialStruct(UnitRange{Int}, ...)`, for the method of
`getindex(::String, r::UnitRange{Int})`. Specifically, the path at
https://github.com/JuliaLang/julia/blob/cebfd7bc66153b82c56715cb1cb52dac7df8eac8/base/compiler/typeinfer.jl#L809-L815
is hit, so the direct cause was the recursion limit for constant
inference.

To explain in more detail, within the slow path of `nextind` which is
called inside `getindex(::String, ::UnitRange{Int})`, the 1-argument
`@assert` is used https://github.com/JuliaLang/julia/blob/cebfd7bc66153b82c56715cb1cb52dac7df8eac8/base/strings/string.jl#L211.
The code related to `print` associated with this `@assert` further uses
`getindex(::String, ::UnitRange{Int})`, causing the recursion limit.
This recursion limit is only for constant inference, which is why we
saw this regression only for the `PartialStruct` case.
Moreover, since this recursion limit occurs within the
`@assert`-related code, this issue did not arise until now (i.e. until
#49260 was merged).

As a solution, I considered improving the recursion limit itself, but
decided that keeping the current code for the recursion limit of
constant inference is safer. Ideally, this should be addressed on the
compiler side, but there is certainly deep recursion in this case. As an
easier solution, this commit resolves the issue by changing the 1-arg
`@assert` to the 2-arg version.

- replaces #55583

base/compiler/typeinfer.jl

@@ -864,7 +864,7 @@ function typeinf_edge(interp::AbstractInterpreter, method::Method, @nospecialize
         end
     end
     if ccall(:jl_get_module_infer, Cint, (Any,), method.module) == 0 && !generating_output(#=incremental=#false)
-        add_remark!(interp, caller, "Inference is disabled for the target module")
+        add_remark!(interp, caller, "[typeinf_edge] Inference is disabled for the target module")
         return EdgeCallResult(Any, Any, nothing, Effects())
     end
     if !is_cached(caller) && frame_parent(caller) === nothing
@@ -897,7 +897,7 @@ function typeinf_edge(interp::AbstractInterpreter, method::Method, @nospecialize
         end
         frame = InferenceState(result, cache_mode, interp) # always use the cache for edge targets
         if frame === nothing
-            add_remark!(interp, caller, "Failed to retrieve source")
+            add_remark!(interp, caller, "[typeinf_edge] Failed to retrieve source")
             # can't get the source for this, so we know nothing
             if cache_mode == CACHE_MODE_GLOBAL
                 engine_reject(interp, ci)
@@ -918,6 +918,7 @@ function typeinf_edge(interp::AbstractInterpreter, method::Method, @nospecialize
         return EdgeCallResult(frame.bestguess, exc_bestguess, edge, effects, volatile_inf_result)
     elseif frame === true
         # unresolvable cycle
+        add_remark!(interp, caller, "[typeinf_edge] Unresolvable cycle")
         return EdgeCallResult(Any, Any, nothing, Effects())
     end
     # return the current knowledge about this cycle

base/strings/string.jl

@@ -208,7 +208,7 @@ end
             i = i′
             @inbounds l = codeunit(s, i)
             (l < 0x80) | (0xf8 ≤ l) && return i+1
-            @assert l >= 0xc0
+            @assert l >= 0xc0 "invalid codeunit"
         end
         # first continuation byte
         (i += 1) > n && return i