[NFC] Move SILFunctionConventions into its own header.

Author: atrick

include/swift/AST/Types.h

@@ -2821,7 +2821,7 @@ class SILParameterInfo {
   /// storage. Therefore they will be passed using an indirect formal
   /// convention, and this method will return an address type. However, in
   /// canonical SIL the opaque arguments might not have an address type.
-  SILType getSILStorageType() const; // in SILType.h
+  SILType getSILStorageType() const; // in SILFunctioConventions.h
 
   /// Return a version of this parameter info with the type replaced.
   SILParameterInfo getWithType(CanType type) const {
@@ -2918,7 +2918,7 @@ class SILResultInfo {
   /// storage. Therefore they will be returned using an indirect formal
   /// convention, and this method will return an address type. However, in
   /// canonical SIL the opaque results might not have an address type.
-  SILType getSILStorageType() const; // in SILType.h
+  SILType getSILStorageType() const; // in SILFunctioConventions.h
 
   /// Return a version of this result info with the type replaced.
   SILResultInfo getWithType(CanType type) const {