Use a larger invalid attribute bitcode number

That way the test won't start faililng when someone adds a new attribute
and wants to use the next logical enum (38) for bitcode.  The new
bitcode file tries to use the number 48 as an attribute instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193787 91177308-0d34-0410-b5e6-96231b3b80d8