[NFC] Move FoldingSetNodeID::AddInteger and FoldingSetNodeID::AddPointer definitions to header

Lack of AddInteger/AddPointer inlining slows down NodeEquals/Profile/:operator== calls.
Inlining makes FunctionProtoTypes/PointerTypes/ElaboratedTypes/ParenTypes Profile functions faster
but since NodeEquals is still called indirectly through function pointer from FindNodeOrInsertPos
there is room for further inlining improvements.

Extracted from: https://reviews.llvm.org/D118385

Differential Revision: https://reviews.llvm.org/D118610