[Frontend] NFC: Remove a mutable SILOptions accessor

We shouldn't expose the ability to mutate the
SILOptions through the CompilerInstance.

Author: hamishknight

include/swift/Frontend/Frontend.h

@@ -494,7 +494,6 @@ class CompilerInstance {
 
   bool hasASTContext() const { return Context != nullptr; }
 
-  SILOptions &getSILOptions() { return Invocation.getSILOptions(); }
   const SILOptions &getSILOptions() const { return Invocation.getSILOptions(); }
 
   Lowering::TypeConverter &getSILTypes();