Reapply "LLVMContext: Store APInt/APFloat directly into the ConstantInt/FP DenseMaps."

This reapplies r223478 with a fix for 32 bit targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223586 91177308-0d34-0410-b5e6-96231b3b80d8