Match llvm-skip-rebuild flag

Author: Matthew Healy

src/bootstrap/flags.rs

@@ -38,6 +38,8 @@ pub struct Flags {
     //
     // true => deny, false => warn
     pub deny_warnings: Option<bool>,
+
+    pub llvm_skip_rebuild: Option<bool>,
 }
 
 pub enum Subcommand {
@@ -495,6 +497,9 @@ Arguments:
                 .map(|p| p.into())
                 .collect::<Vec<_>>(),
             deny_warnings: parse_deny_warnings(&matches),
+            llvm_skip_rebuild: matches.opt_str("llvm-skip-rebuild").map(|s| s.to_lowercase()).map(
+                |s| s.parse::<bool>().expect("`llvm-skip-rebuild` should be either true or false"),
+            ),
         }
     }
 }