Emit an error if -Zdwarf-version=1 is requested

DWARF 1 is very different than DWARF 2+ (see the commentary in
https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#index-gdwarf)
and LLVM does not really seem to support DWARF 1 as Clang does not offer
a `-gdwarf-1` flag and `llc` will just generate DWARF 2 with the version
set to 1: https://godbolt.org/z/s85d87n3a.

Since this isn't actually supported (and it's not clear it would be
useful anyway), report that DWARF 1 is not supported if it is requested.

Also add a help message to the error saying which versions are supported.

compiler/rustc_session/src/errors.rs

@@ -161,6 +161,7 @@ pub(crate) struct UnstableVirtualFunctionElimination;
 
 #[derive(Diagnostic)]
 #[diag(session_unsupported_dwarf_version)]
+#[help(session_unsupported_dwarf_version_help)]
 pub(crate) struct UnsupportedDwarfVersion {
     pub(crate) dwarf_version: u32,
 }