The necessary changes were only in upstream LLVM in 4.0, but they were
for a while backported to Rust LLVM. Now that Rust LLVM is also 4.0, we
can make the test conditional here more accurate.
This changes rustc to emit DW_AT_main_subprogram on the "main" program.
This lets gdb suitably stop at the user's main in response to
"start" (rather than the library's main, which is what happens
currently).
Fixes#32620
r? michaelwoerister
