Auto merge of #38109 - tromey:main-subprogram, r=michaelwoerister

Emit DW_AT_main_subprogram

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

diff --cc src/librustc_llvm/ffi.rs
Simple merge

diff --cc src/librustc_trans/debuginfo/mod.rs
Simple merge

diff --cc src/rustllvm/RustWrapper.cpp
Simple merge