Support transient in StorageLocation (#269)

Trivial patch. The new variant `transient` seems missing.

Author: wtdcode

crates/artifacts/solc/src/ast/misc.rs

@@ -92,6 +92,7 @@ pub enum StorageLocation {
     Default,
     Memory,
     Storage,
+    Transient,
 }
 
 /// Visibility specifier.