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

Required some APInt massaging to get proper empty/tombstone values. Apart
from making the code a bit simpler this also reduces the bucket size of
the ConstantInt map from 32 to 24 bytes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223478 91177308-0d34-0410-b5e6-96231b3b80d8