Revert r51498:...

Revert r51498: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDecl.cpp?rev=51498&r1=51497&r2=51498&view=diff

Turns out that there are multiple places where a redefinition diagnostic can be emitted.  A cleaner solution (without touching Sema) is to have
the Driver turn off these diagnostics.  (will submit this patch soon)

llvm-svn: 51502