Merge pull request #84180 from meg-gupta/borrowandmutatepr

Add preliminary support for borrow accessors

lib/SILGen/SILGenPattern.cpp

@@ -3357,6 +3357,10 @@ static bool isBorrowableSubject(SILGenFunction &SGF,
     case AccessorKind::WillSet:
     case AccessorKind::DidSet:
       llvm_unreachable("should not be involved in a read");
+    case AccessorKind::Borrow:
+      llvm_unreachable("borrow accessor is not yet implemented");
+    case AccessorKind::Mutate:
+      llvm_unreachable("mutate accessor is not yet implemented");
     }
     llvm_unreachable("switch not covered?");