CodeGen: Don't crash when replacing functions

The peculiarities of C99 create scenario where an LLVM IR function
declaration may need to be replaced with a definition baring a different
type because the prototype and definition are not required to agree.

However, we were not properly deferring this when it occurred.

This fixes PR19280.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205099 91177308-0d34-0410-b5e6-96231b3b80d8