[Kaleidoscope] toy.cpp use after move fix

[Kaleidoscope] toy.cpp use after move fix

The variable Proto is moved at the beginning of the codegen() function.
According to the comment above, the pointed object should be used due the
reference P.

Differential Revision: https://reviews.llvm.org/D32939

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302369 91177308-0d34-0410-b5e6-96231b3b80d8