Revert "[CodeGen] Fix crash when a function taking transparent union is redeclared."

This reverts commit r321296. It caused performance regressions
FAIL: imp.execution_time
FAIL: 2007-01-04-KNR-Args.execution_time
FAIL: sse_expandfft.execution_time
FAIL: sse_stepfft.execution_time

llvm-svn: 321306