[NFC] Remove TypeChecker::getSubstringType

Inline it into its only caller

Author: CodaFi

lib/Sema/CSDiagnostics.cpp

@@ -2571,10 +2571,7 @@ bool ContextualFailure::trySequenceSubsequenceFixIts(
     return false;
 
   auto String = TypeChecker::getStringType(getASTContext());
-  auto Substring = TypeChecker::getSubstringType(getASTContext());
-
-  if (!String || !Substring)
-    return false;
+  auto Substring = getASTContext().getSubstringDecl()->getDeclaredInterfaceType();
 
   // Substring -> String conversion
   // Wrap in String.init

lib/Sema/TypeCheckType.cpp

@@ -387,10 +387,6 @@ Type TypeChecker::getStringType(ASTContext &Context) {
   return Type();
 }
 
-Type TypeChecker::getSubstringType(ASTContext &Context) {
-  if (auto typeDecl = Context.getSubstringDecl())
-    return typeDecl->getDeclaredInterfaceType();
-
   return Type();
 }