[NFC][Clang] Remove redundant function definitions

There were 3 definitions of the mergeDefaultFunctionDefinitionAttributes
function: A private implementation, a version exposed in CodeGen, a
version exposed in CodeGenModule.

This patch removes the private and the CodeGenModule versions and keeps
a single definition in CodeGen.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D159256