core/codegen: Error if a function is redeclared.
core/codegen: Error if a function is redeclared.

This was never an error as LLVM would append numbers to the end of the function name making these unique. In reality, duplicated functions is probably unintended, so make it an error.

google/gapid,google/gapid,google/agi,pmuetschard/gapid,google/gapid,Qining/gapid,google/gapid,google/agi,google/agi,Qining/gapid,pmuetschard/gapid,pmuetschard/gapid,google/gapid,Qining/gapid,Qining/gapid,pmuetschard/gapid,google/agi,Qining/gapid,Qining/gapid,pmuetschard/gapid,google/agi,google/gapid,google/agi,pmuetschard/gapid,google/gapid,Qining/gapid,google/agi,pmuetschard/gapid,pmuetschard/gapid,Qining/gapid,google/agi,google/gapid