Fix another IRBuilder InsertionPoint leak.

[maleadt]

Similar to #18054. The builder used in gen_cfun_wrapper sets but doesn't clear its insertion point, which can get destroyed during finalization. When doing old = builder.GetInsertionPoint(); emit_whatever(); builder.SetInsertionPoint(old) later on, LLVM reads from that invalid memory.

[tkelman]

is this detected by existing tests when run in a sanitizer configuration?

[maleadt]

Yes, I caught it in the ccall tests. Not always though, and I didn't figure out the exact interplay to figure out an good, exact test.

[maleadt]

Uhh

Error During Test
  Test threw an exception of type Base.UVError

  Expression: wait(watch) == Base.Filesystem.FileEvent(false,false,true)
  start_watching (FileMonitor): no space left on device (ENOSPC)

Restarted CI.