[FIRRTL] Chang typedef to using, NFC

Change a `typedef foo bar` to a `using bar = foo` in the Grand Central
pass.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

lib/Dialect/FIRRTL/Transforms/GrandCentral.cpp

@@ -520,7 +520,7 @@ struct VerbatimType {
 
 /// A sum type representing either a type encoded as a string (VerbatimType)
 /// or an actual mlir::Type.
-typedef std::variant<VerbatimType, Type> TypeSum;
+using TypeSum = std::variant<VerbatimType, Type>;
 
 /// Stores the information content of an ExtractGrandCentralAnnotation.
 struct ExtractionInfo {