Allow separation of declarations and definitions in <Target>ISelDAGToDAG.inc

This patch adds the ability to include the member function declarations
in the instruction selector class separately from the member bodies.

Defining GET_DAGISEL_DECL macro to any value will only include the member
declarations. To include bodies, define GET_DAGISEL_BODY macro to be the
selector class name. Example:

  class FooDAGToDAGISel : public SelectionDAGISel {
    // Pull in declarations only.
    #define GET_DAGISEL_DECL
    #include "FooISelDAGToDAG.inc"
  };

  // Include the function bodies (with names qualified with the provided
  // class name).
  #define GET_DAGISEL_BODY FooDAGToDAGISel
  #include "FooISelDAGToDAG.inc"

When neither of the two macros are defined, the function bodies are emitted
inline (in the same way as before this patch).

Differential Revision: https://reviews.llvm.org/D39596

llvm-svn: 317903