[clang][NFC] Annotate CGRecordLayout.h with preferred_type

This helps debuggers to display values in bit-fields in a more helpful way.

Author: Endilll

clang/lib/CodeGen/CGRecordLayout.h

@@ -71,6 +71,7 @@ struct CGBitFieldInfo {
   unsigned Size : 15;
 
   /// Whether the bit-field is signed.
+  LLVM_PREFERRED_TYPE(bool)
   unsigned IsSigned : 1;
 
   /// The storage size in bits which should be used when accessing this