[AST][NFC] Various cleanups to GenericSelectionExpr

Various cleanups to GenericSelectionExpr factored out of D57104. In particular:

1. Move the friend declaration to the top.
2. Introduce a constant ResultDependentIndex instead of the magic "-1".
3. clang-format
4. Group the member function together so that they can be removed as one block
   by D57106.

NFC.

Differential Revision: https://reviews.llvm.org/D57238

Reviewed By: aaron.ballman

llvm-svn: 352275