compiler: don't warn for print()

compiler: don't warn for print()

We used to warn for calls to print(), because it doesn't do anything.
However, a Go 1.18 test uses that call, and it is valid Go.  Change
the compiler to just accept it and compile it; this will produce calls
to printlock and printunlock, and nothing else.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/384355