make Core.OpaqueClosure(ir::IRCode) non-destructive (#49100)

Currently the constructor is destructive because it calls
`replace_code_newstyle!(src, ir, ...)` etc.
This commit made it non-destructive by calling `Core.Compiler.copy(ir)`
beforehand. We can define a destructive version of this
`Core.OpaqueClosure!` later if we really want it.

base/opaque_closure.jl

@@ -63,6 +63,7 @@ function Core.OpaqueClosure(ir::IRCode, env...;
  if (isva && nargs > length(ir.argtypes)) || (!isva && nargs != length(ir.argtypes)-1)
  throw(ArgumentError("invalid argument count"))
  end
+ ir = Core.Compiler.copy(ir)
  src = ccall(:jl_new_code_info_uninit, Ref{CodeInfo}, ())
  src.slotflags = UInt8[]
  src.slotnames = fill(:none, nargs+1)

test/opaque_closure.jl

@@ -256,8 +256,7 @@ let src = first(only(code_typed(+, (Int, Int))))
  @test oc(40, 2) == 42
  @test isa(oc, OpaqueClosure{Tuple{Int,Int}, Int})
  @test_throws TypeError oc("40", 2)
- ir = Core.Compiler.inflate_ir(src)
- @test OpaqueClosure(ir)(40, 2) == 42
+ @test OpaqueClosure(ir)(40, 2) == 42 # OpaqueClosure constructor should be non-destructive
 end
 
 # variadic arguments