Save another call to GetAddrOfFunction.

Thread an optional GV down to EmitGlobalFunctionDefinition so that it can
avoid the lookup when we already know the corresponding llvm global value.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196789 91177308-0d34-0410-b5e6-96231b3b80d8