Add -Z proc-macro-backtrace to allow showing proc-macro panics

Fixes #75050

Previously, we would unconditionally suppress the panic hook during
proc-macro execution. This commit adds a new flag
-Z proc-macro-backtrace, which allows running the panic hook for
easier debugging.

compiler/rustc_session/src/options.rs

@@ -967,6 +967,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "print the result of the monomorphization collection pass"),
     print_type_sizes: bool = (false, parse_bool, [UNTRACKED],
         "print layout information for each type encountered (default: no)"),
+    proc_macro_backtrace: bool = (false, parse_bool, [UNTRACKED],
+         "show backtraces for panics during proc-macro execution (default: no)"),
     profile: bool = (false, parse_bool, [TRACKED],
         "insert profiling code (default: no)"),
     profile_emit: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],