[NFC][LLVMContext] Clean up DenseMapInfo classes used for APInt & APFloat.

DenseMapAPIntKeyInfo looks like a redundant definition because it
mirrors the default used by DenseMap when not specified.

Replacing DenseMapAPFloatKeyInfo with a specialisation of
DenseMapInfo allows DenseMap<T> to be more easily used when T is
an aggregate type containing an APFloat.