Revert "Add a new insert_as() method to DenseMap and use it for ConstantUniqueMap"

This reverts commit r260458.

It was backported on an internal branch and broke stage2 build. Since
this can lead to weird random crash I'm reverting upstream as well
while investigating.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 260605