Fix argument alias handling for -g and -C debuginfo

Author: varkor

src/librustc/session/config.rs

@@ -2217,10 +2217,15 @@ pub fn build_session_options_and_crate_config(
         }
     };
     let debug_assertions = cg.debug_assertions.unwrap_or(opt_level == OptLevel::No);
-    let debuginfo = if matches.opt_present("g") {
-        if cg.debuginfo.is_some() {
-            early_error(error_format, "-g and -C debuginfo both provided");
+    let max_g = matches.opt_positions("g").into_iter().max();
+    let max_c = matches.opt_strs_pos("C").into_iter().flat_map(|(i, s)| {
+        if let Some("debuginfo") = s.splitn(2, '=').next() {
+            Some(i)
+        } else {
+            None
         }
+    }).max();
+    let debuginfo = if max_g > max_c {
         DebugInfo::Full
     } else {
         match cg.debuginfo {